Emil, do you use third-party plugins?  Did you try to deactivate all third-party plugins to exclude them?
You may also check ow_base_cron_job database table. This database tables stores information related to Cron Jobs’ activity on your Oxwall website.

Thanks for the reply dear Taissa :)

Yes, even with system plugins only site gets slow. Site is rather small and doesn't even have many users online. My hosting provider says I don't even hit 10% of the resources of my hosting plan so it's not any limits imposed by the host. They say it must be the software itself I just can't figure out what the problem is... during daytime it runs completely fine, during 18-22 hr. it gets sometimes impossible to load.

No sure what to look for in ow_base_cron_job (sorry I'm a complete idiot)

Here's a screenshot

look mail logs in whm maybe thats time most emails are being sent 
maybe the server is set to store mails and send at that time 

but you not say what hosting are you on 
are you on shared server ?

Yes shared hosting (highest plan) 90 gb storage 180 min cpu time, 1.5 GB ram 3 MB/s i/o usage but the weird thing is I don't use even 10% of recources here's stats from cpanel

How to do that in CPanel? (track mails sent?)

when your site is slow whats ur cpu etc
being on shared maybe it not even you but other sites using all the server resources 

and other sites using all the bandwidth 

Hey Tammy, thanks for your replies. When site is slow or not loading CPU is still under 10% and resources do not hit any limit. Also it is not the server because I already switched 3 servers because of this problem (thankfully my hosting provider is patient with me) and I tried other sites on that server and they load just fine at the time my site doesn't load.

The weird part is that most of the time it loads alright, even fast... but there are times usually evening (18-22 hr) that it starts to lag seriously.

Here's a Profiler dump. (I don't know how to read that). But this dump is from when the site loads okay because I can't get one from when the site is slow because it just doesn't show up.

Emil, to exclude the cron job issue please set the cron runs once a week in cPanel  for one day, for example. And check if the issue still persist.
Also, hosting can provide you with the trace of queries that load your server the most. And if the issue is connected to cron you will see the .../ow_cron/run.php in this trace.