Thanks for your reply!

Unfortunately this happens with version 6.22.
I managed to get smooth gameplay by having only about half of the entities in the level ( about 500 ) and ent_creating the rest and removing them at runtime when needed so that the count never gets over 500. This way (along with avoiding ent_create peaks) I achieved completely smooth gameplay, that was not at all possible when having all the entities in the level from the start.
How many entities there can be before the visible slowdown appears seems to be related ( though I might be wrong ) to CPU performance.

It's great to hear that the slowdown problem in 6.30 was located.
I'm looking forward to trying the improved statistics panel! What do you exactly mean by "next version"? The next public beta or 6.4?