Drive caches are pointless these days since all operating systems released in the last 10+ years have done their own caching with main system ram. There is no point in the drive caching things that the OS already has cached ( and thus, won’t ask for again ). The slow down you describe sounds like normal Windows stupidity causing heavy fragmentation.
Games generally read whatever data they will need when loading a level or similar; they don’t write a bunch of data to disk and then try to read other data that they need immediately and thus, can not continue to play smothly. Game designers figured out long ago how to do seamless zone transitions by reading new map data when you get close to a seamless border so it is availible when you cross the border and the data is actually needed. Operating systems also try to flush writes to the disk slowly in the background so that reads remain responsive.
