Why does redis work better with maxmemory than without it?

Question

Redis

Good afternoon.
Faced such a problem. There is an application that searches, works with redis. (It doesn't matter what it looks for.) So, if redis is set to maxmemory 1000mb and maxheap 1500mb, then the application works stably and searches quickly. And if these parameters are increased, then the application starts to "sag".
For example, there were 1000, 1200, 1100, 1000, etc. searches/minute. And if you increase maxmemory and maxheap, then the values ​​are 1000, then 500, then 1500, etc.
I don't know which way to dig. It seems that redis keeps everything in memory and the more allocated to it, the better it should be.

Answer 1

Doesn't it go to swap by any chance? Well, how do you measure "subsidence"?

Answer 2

Should not. The server has 24 gigabytes of RAM. Even with maxmemory 4 GB, there are sagging. We have a windows counter showing the number of searches per unit of time. Well, as I said, the schedule starts jumping like hell.