@royal_man to ur use case talking above, only file-system-cache can help.
Just change or append the index setting, with enough free mem in ur data nodes (to our-tsdb-use-case: 31G on-heap + 96G off-heap / per node):
index.store.preload: ["nvd", "dvd", "tim", "doc", "dim"]
or more aggressive
index.store.preload: ["*"]
And reopen ur index, u will see difference!
Wish it can help u!