We expect about 90% of data to be in the highest level of LCS in a fully populated series. (See also CASSANDRA-9829.)
Thus if the user is primarily asking for data (partitions) that has actually been inserted, the bloom filter on the highest level only helps reject sstables about 10% of the time.
We should add an option that suppresses bloom filter creation on top-level sstables. This will dramatically reduce memory usage for LCS and may even improve performance as we no longer check a low-value filter.
(This is also an idea from RocksDB.)