If consumers are keeping up, there will be no reads to the BookKeeper layer as the Pulsar broker will serve from memory.

When reads need to go to BookKeeper there are caches there too, with read-aheads to populate the cache to avoid going back to disk regularly.

Even when having to go to disk, there are further optimizations in how data is laid out on disk to ensure as much sequential reading as possible.

Also note that the fragments aren't necessarily that small either.