Dashboard Unusably Slow – Front End Fine

I know this question has been referenced in the past, but I haven’t seen anything up-to-date, or containing a fully comprehensive answer. I just want to say thanks in advance for any help you might be able to offer me.

Cutting to the chase: I inherited a site that is very poorly optimized and I’m doing what I can to get it working properly. The front end of this site seems to be running fine when I try to navigate in various browsers including incognito mode. The dashboard, however, is basically unusable. When I CAN get into the dashboard, it’s taking MINUTES to click through to anything or to update anything. Other times I’ll get shot straight to a 500 Internal Server Error. The problems will sometimes affect the front end, but for the most part we manage to remain functional, sans the occasional 500 error which I assume is related to the same issue.

The Kicker: The problems are intermittent. At times, the site will work perfectly and I’ll be able to navigate through the dashboard with finesse. I’m beginning to think it might be server side, and that the slow times might be correlated with times of high traffic to the site, but as for now that’s just speculation.

I’m basically at a loss. I will tell you that the site is very heavily edited, but mostly in css, and the PHP hasn’t been touched that much minus the addition of a few sidebars, adding a custom header image, redoing the footer, etc (basically nothing that would be kicking super hard errors).

I’m a marketing guy, and at this point, I’m somewhat out of my depth and would appreciate any help anyone could offer me as to what might be going on.

Site for reference: https://allotsego.com