Percona/XtraDB Database with many of writes to a small table uses less CPU than the same database with less writes?
In all versions (Percona, XtraDB, MariaDB) of InnoDB, writes and index updates are delayed.  A separate thread(s) takes care of doing the writes 'eventually' and in the 'background'.

Data writes are put into the desired blocks immediately, but not written until they get bumped out of the buffer_pool.

UNIQUE indexes are updated immediately but left cached in the buffer_pool (like data).

Non-unique indexes are stored in the "Change buffer".  Later they are collected together for putting into the actual index blocks and eventually written to disk.

Redo log and double-write writes are done more immediately.

There are many settings relating to the frequency of background actions, sizes of caches, etc.  These have an impact on both CPU and I/O.  Normally it is best to leave them alone.

To further address your question, please provide the RAM size of the servers being tested and a diff of the SHOW VARIABLES.  The diff may help zoom in on the cause of the difference you are seeing.