You never heard of a profiler? Logging? You're going about this all wrong.
When fixing performance problems you shouldn't guess, just profile it to find the bottlenecks.
I've seen plenty of performance 'fixes' that weren't, pure guesses by developers that did nothing, when a quick profile immediately revealed the culprit.
In your case you also need to figure out if it's happening server-side or client-side. I generally start with the server-side logs, get a few days/weeks worth of data, find average page request times, plus how much deviation on those requests, then go from there. That gets you the server-side. For client-side, unfortunately it's a lot harder. Google analytics page load speed, for example, is a pile of crap. But, again, there's a profiler in dev tools, remember js compile time is a significant thing and can slow load time too so check that out as well as the actual run times (js compile time shows in the page load graph).
When fixing performance problems you shouldn't guess, just profile it to find the bottlenecks.
I've seen plenty of performance 'fixes' that weren't, pure guesses by developers that did nothing, when a quick profile immediately revealed the culprit.
In your case you also need to figure out if it's happening server-side or client-side. I generally start with the server-side logs, get a few days/weeks worth of data, find average page request times, plus how much deviation on those requests, then go from there. That gets you the server-side. For client-side, unfortunately it's a lot harder. Google analytics page load speed, for example, is a pile of crap. But, again, there's a profiler in dev tools, remember js compile time is a significant thing and can slow load time too so check that out as well as the actual run times (js compile time shows in the page load graph).