Cassandra saves a sample of IndexInfo objects that store the offset within each partition of every 64KB (by default) range of rows. To find a row, we binary search this sample, then scan the partition of the appropriate range.
The problem is that this scales poorly as partitions grow: on a cache miss, we deserialize the entire set of IndexInfo, which both creates a lot of GC overhead (as noted in CASSANDRA-9754) but is also non-negligible i/o activity (relative to reading a single 64KB row range) as partitions get truly large.
We introduced an "offset map" in
CASSANDRA-10314 that allows us to perform the IndexInfo bsearch while only deserializing IndexInfo that we need to compare against, i.e. log(N) deserializations.