When node is doing a lot of sequencial IO (streaming, compacting, etc) a lot of CPU is lost in calls to RAF's int read() and DataOutputStream's write(int).
This is because default implementations of readShort,readLong, etc as well as their matching write* are implemented with numerous calls of byte by byte read and write.
This makes a lot of syscalls as well.
A quick microbench shows than just reimplementation of these methods in either way gives 8x speed increase.
A patch attached implements RandomAccessReader.read<Type> and SequencialWriter.write<Type> methods in more efficient way.
I also eliminated some extra byte copies in CompositeType.split and ColumnNameHelper.maxComponents, which were on my profiler's hotspot method list during tests.
A stress tests on my laptop show that this patch makes compaction 25-30% faster on uncompressed sstables and 15% faster for compressed ones.
A deployment to production shows much less CPU load for compaction.
(I attached a cpu load graph from one of our production, orange is niced CPU load - i.e. compaction; yellow is user - i.e. not compaction related tasks)