From #openttdcoop wiki
This document aimes to enable players to get the most out of YETI Extended Towns and Industries. This document assumes that you've already downloaded and installed YETI from the online content downloader.
Optimizing Playing Environment
Because YETI uses high number of animated frames, the games may run sluggish if played with default Openttd configuration. You can try one of these configuration changes if you find the YETI game running sluggish.
Option 1: Increasing sprite cache size
Locate your copy of openttd.cfg file. Under the [misc] section of the file (it should be the first section in the file), you should already have sprite_cache_size_px. Set its value to 256 or larger. I personally have it set it to 384. Also you can add these lines too:
sprite_cache_size = 512 max_sprite_cache_size = 512
If Openttd gives you an error about unable to allocate memory required, then you can try option 2.
Option 2: Switch from 32bpp to 8bpp mode
Nowadays, Openttd runs in 32 bit per pixel color mode, as modern games should. But you can configure Openttd to run in 8 bit color mode. In your openttd.cfg file, under [misc] section of the file, you should already have blitter setting, even though the value may be empty. Set this line to
blitter = "8bpp-optimized"
When you load a YETI game with this setting, YETI will use non-animated version of graphics, and it should lower the memory needed to run YETI game smoothly.