There are some issues with how the UI and world map are rendered, and the rendering resources are managed:

• Rendering can affect game world. Main offender is the memory map, which is added to mostly on rendering stage, but is used for some in-game functionality like vehicle pathfinding. There's also this: Game effects are applied during drawing #993
• Game definitions such as items, creatures, terrain, etc. don't cache graphical tiles they're using. Meaning, on every frame for every visible thing on the map the game has to look up the needed graphical tile, and on failure, resolve the looks-like chain to find the closest match.
• The map is drawn from scratch on every frame, but situations when that's needed are very rare. Most of the time, contents of the frame are almost fully identical to contents of the previous frame.
• Graphical tile animations cause full redraws.
• Graphical tile animations halt when player opens a UI that does not explicitly call for map view redraws (e.g. all "Do action where?" prompts)
• There are multiple independent animation systems in place: SCT (scrolling combat text), weather particles, explosion animations, bullet animations, graphical tile animations.
• Ids of graphical tiles used by the UI are hardcoded, and are not easily available for tileset makers.
• There can only be 1 type of every "overlay" at once, meaning complex UIs (e.g. aiming UI that displays multiple lines of fire) have to hack around this restriction.
• It is impossible to mix interface and graphical tiles
• Map/overmap viewports are hardwired