As Anders says, it is actually impossible to answer, also because the setups and requirements in the Checkmk userbase are so widely different. As you are looking for user experiences, here is something I remember:
In general, we are always working on improving performance of Checkmk. Thus, every version has to be better than the one before. Just due to the fact that we have pretty large installations out there with millions of services and hundreds of users - and if we wouldn’t try to improve the performance continuously that wouldn’t work out.
As a recap, there were a couple of major changes in recent versions.
In Checkmk 2.0, a major piece was rearchitected: the helpers were split into fetchers and checkers.
Also activate changes was improved in 2.0 with incremental sync of configs of remote sites (no more full config sync). Activate changes was further reworked in 2.1 to activate typical changes (like adding hosts) much quicker.
Also the search bars in Checkmk 2.0 are leveraging Redis, and thus are pretty quick.
We also went from Python 2.7 to Python 3.8 in Checkmk 2.0, which probably didn’t do much in performance. But rumors are Python 3.11, which will be shipped with Checkmk 2.2 is much faster.
There are many further changes, like adding caches here and there, which can have a lot of impact.