Alex, I'm still looking into your question.
I am afraid that we will not be able to resolve this issue in a short time.
For now, I'm leaning towards the fact that you have some kind of heavy background process. For example, cron hour from onebox, well, or even a non-OneBox process, for example, a backup of something. When this process is started (I suppose once an hour or once every couple of hours), then it forces out some of the data from the RAM to swap. And then the operating system begins to gradually shift everything back to the operative, but on demand. That is, the user enters some page, data is required, and they are in swap - bang, everything is stupid, and from the next page update it is gone (because the operating system figured out to shift everything to ram).
I previously described how this RAM vs SWAP mechanism works here -
https://crm-onebox.com/ru/support/server-installation/1423-swap-na-serveri-prakt...How to solve such a question (if this is it):
find a process that briefly devours the RAM and do something with it:
a) turn off, optimize, run less often.
b) add RAM to level swap swaps.
And it's still not clear which option is cheaper and faster :)