When a snasphot is initially loaded, the snapshot chain gets most of the cache to catch up to the tip as fast as possible. When it leaves IBD, we can then allocate most of the cache again to the background IBD chain.
This is a partial backport of core#27596
https://github.com/bitcoin/bitcoin/pull/27596/commits/9f2318c76cc6986d48e13831cf5bd8dab194fdf4