Tests showed that doing multiple set() into session cache takes enourmous amount of time and RAM.
cache::make() returns instance of cache_session.
cache_session::set() adds the new key to the $this->session unconditionally (without checking any maxsizes).
Then during the call to cache_sessionstore::set() the key is actually the sessionid and the data is the whole session. So there is always only one key and all checks for maxsize are useless.

This means that doing the second call to $cache->set() will write both first and second data. Calling set() for the third time will write first, second and third, and so on. No wonder tests show that average time for set() call is increasing when number of calls increases.