Optimising Foundry for Large Encounters

The city guard are breaking. A tide of undead surges through the market square, the cobblestones slick with rain and ichor. Dozens of tokens—allies, enemies, civilians—are locked in a desperate melee under the flickering gas lamps of a failing city. The scale is epic, the stakes are high… and your player’s frame rate just dropped to a crawl.

Foundry VTT is a peerless platform for creating immersive, dynamic TTRPG experiences. Its power, however, is a double-edged sword. With great customisability comes the potential for performance bottlenecks, especially when your ambitions grow to encompass vast cityscapes or truly massive battles. But fear not, Game Master.

This guide is your grimoire of techniques for mastering Foundry VTT optimisation. We will delve into the arcane arts of scene configuration, asset management, and module curation to ensure your most ambitious encounters run as smoothly as a rogue’s infiltration.

The Foundation: Core Settings & Scene Configuration

Before you start culling your favourite modules or compressing every asset into oblivion, the first place to look for performance gains is within Foundry’s core settings. How you build your scenes has the single greatest impact on how they perform for you and your players.

Mastering Scene Dimensions

A common mistake is creating scenes with colossal pixel dimensions to accommodate a high-resolution map image. Foundry VTT has to render the entire scene, including the padded ‘out of bounds’ area, even if players can only see a fraction of it. A 10,000 by 10,000 pixel scene taxes the system far more than a 4,000 by 4,000 pixel one, regardless of the visible area.

When configuring a new scene, pay close attention to the background image dimensions. Aim to keep your map files as small as possible without sacrificing visual clarity. A good rule of thumb is to use a grid size of between 70 and 150 pixels per square. Any higher often yields diminishing returns for a significant performance cost. Set your scene dimensions to match your map image, and use a modest ‘Padding Percentage’ (5-10%) to give yourself a little breathing room around the edges.

The Power of WebP

Your choice of image format is critical. While JPGs are common and PNGs offer transparency, the WebP format is the undisputed champion for VTT assets. It provides excellent compression, often creating files significantly smaller than JPG or PNG equivalents, while supporting transparency and maintaining high visual fidelity.

Converting your maps, tokens, and tile assets to WebP before uploading them is a foundational step in Foundry VTT optimisation. Tools like GIMP, Photoshop (with a plugin), or numerous online converters can handle this process. A smaller file size means faster loading for your players and less memory usage for everyone, which is paramount for large battle maps.

Taming Walls and Lights

Foundry’s dynamic lighting system is a marvel, but every wall and every light source adds to the computational load. A complex dungeon with hundreds of individual wall segments will perform worse than one with longer, continuous walls.

When drawing walls, try to use as few segments as possible to create the desired shape. For complex curves, it is often better to approximate with a series of straight lines than to draw dozens of tiny segments. Furthermore, be judicious with light sources. A scene with fifty flickering torches, each calculating its own radius and casting dynamic shadows, will bring even powerful systems to their knees. Use fewer, larger light sources where possible, and consider using the ‘Unrestricted’ wall type for decorative elements that don’t need to block vision, as this reduces calculations.

Asset Management: The Art of a Lean Game World

Your game world is built from assets: maps, tokens, tiles, and sounds. The size and efficiency of these assets directly influence your world’s loading times and in-game performance. A lean, well-managed asset library is the hallmark of a high-performance Foundry setup.

Pre-processing Your Maps

Never upload a massive, multi-megabyte map file directly from a map-making tool or artist’s page. You must pre-process your maps. This involves two key steps: resizing and compressing.

First, resize the image to your target resolution. As mentioned, 100 pixels per grid square is a fantastic sweet spot for most games. If a map is 50 squares wide, its image width should be around 5,000 pixels. Second, compress the image. Use your image editor’s ‘Save for Web’ function or a dedicated compression tool to reduce the file size, converting it to WebP in the process. Your goal should be to get most battle maps under 2-3 MB. For truly large battle maps, even 5-8 MB is acceptable if the dimensions require it, but every kilobyte saved helps.

The Perils of Animated Assets

Animated maps, tokens, and spell effects are visually stunning, but they are performance vampires. A single animated WebM map background is effectively a video that every connected player’s computer must constantly decode and render. This consumes significant CPU and GPU resources.

Use them sparingly, as a centrepiece for a truly special encounter rather than the norm. If a player is experiencing lag, the first thing to check is often an animated asset. Sometimes, a high-quality static image with a few well-placed tile effects can create a similar atmosphere with a fraction of the performance cost.

Compendiums are Your Allies

When you launch your Foundry world, it loads all the Actors, Items, and Journal Entries that exist in the sidebars. If you have hundreds of monster stat blocks sitting in your Actors tab, you are increasing your world’s loading time and memory footprint unnecessarily. The solution is simple: use compendiums.

Drag any actor, item, or journal entry that you do not need for the immediate session into a compendium pack. The data is stored safely but is not actively loaded into memory until you pull it out of the compendium. A well-organised compendium structure is not just good for organisation; it is a critical tool for maintaining a snappy, responsive game world.

The Module Ecosystem: Curation, Not Collection

Modules are the lifeblood of the Foundry VTT experience, but they are also the most common source of performance degradation. A bloated module list, full of outdated or conflicting code, can grind your game to a halt. Treat your module list like a specialist’s toolkit, not a collector’s hoard.

Conducting a Module Audit

Periodically, you must perform a module audit. Deactivate all non-essential modules and see how your game performs. Then, re-enable them one by one to identify any that cause a noticeable slowdown. The module ‘Find the Culprit’ is an excellent diagnostic tool that can help automate this process.

Be ruthless. If you haven’t used a module in months, disable it. If a module provides a minor cosmetic tweak but you suspect it’s causing lag, disable it. A lean, curated module list is essential for a stable and performant game, especially when dealing with crowded encounters where many different module effects might be firing at once.

Server-Side vs. Client-Side Load

It is useful to understand that modules can impact performance in two ways. Server-side modules add load to the host machine (either the GM’s PC or a dedicated server). These are often modules that handle automation, complex calculations, or data management. Client-side modules primarily affect the players’ browsers, dealing with animations, UI changes, and visual effects.

If only one player with an older machine is lagging, the issue is likely a client-side module. If everyone, including the GM on a powerful PC, experiences slowdowns simultaneously, a server-side module is a more likely suspect. This distinction can help you narrow down your search for the performance bottleneck.

Hardware and Hosting Considerations

Finally, the hardware running the game cannot be ignored. While optimisation can achieve wonders, there are physical limits to what a machine can handle.

If you are self-hosting, your PC is acting as both the server and your own game client. This is demanding. A capable CPU and at least 16GB of RAM are recommended for hosting complex Foundry games. Ensure your players are also taking basic steps, such as using a modern browser like Chrome or Firefox with hardware acceleration enabled, and closing unnecessary background applications.

For the ultimate in performance and reliability, consider a dedicated hosting service. These platforms run your Foundry server on hardware specifically optimised for the task. This offloads all the server-side work from your computer, freeing up its resources to simply run the game as a client. It also provides a more stable connection for all players and is often the best solution for GMs planning truly epic-scale campaigns.

By methodically applying these principles—building efficient scenes, managing your assets wisely, curating your modules, and understanding your hardware’s role—you can transform Foundry VTT from a potential source of frustration into a seamless gateway to the incredible stories you want to tell. Your epic encounters deserve to be remembered for their thrilling narrative, not for their technical difficulties.