I am having this problem too.
IIRC*, 4.1.2 update was fine. In fact, it seemed less SLOW than previous versions had been.
By time I went to do the 4.13 update, it was already up to 4.4. So I updated to 4.1.4.
I now have 4.1.5 and it is the slowest ever.
I do not know if this SUDDEN EXTREME SLOWNESS was introduced in 4.13 or 4.14, but WOW!
It is now PAINFUL to use. Loading ANY page on front end or back end takes extremely long. Editing a Pro page takes a while.
(*(I may be 1 update off somewhere in my recollections, but I am currently on 4.1.5, the latest version.))
I have even closed almost everything on my computer. I usually have an absurd number of browser tabs (usually dozens of windows, hundreds of tabs), XYPlorer (AMAZING file browser!!!), Sublime text, MySQL Workbench, Git Bash console, various file comparison software, etc. (and WAMP) running at once.
Now I have ONLY Notepad++, and 6 TABS in a SINGLE browser window, and WAMP (else I could not work on the website al all).
I have rebooted several times.
I have disabled every plugin that I do not absolutely need to have enabled the specific task I am working on.
The difference is DRAMATIC. Click, wait. Reload, wait. etc.
Also, there has LONG been an issue with MENUS.
Editing Menus when Pro is active is even WORSE than the current SLOWNESS issue.
Literally minutes will pass after loading the page, and being able to click on anything.
Literally minute(s) between any click and responsiveness. And trying to drag a menu item to a different location: good luck, and a LOT of patience.
Even have to wait minute(s) before being able to save.
At one point, I decided it was more productive to recreate all menus, rather then edit/re-order, change pages listed. So I deleted all menus I have created for use with Pro theme. Rebuilt them, and it seemed to work ok. for a couple days/edits.
But now, it is just as painful as before. I am not sure of an earlier update fixed the Menus issue, and the Latest (1-2) update(s) broke it again. Or if Menus when Pro is active can only be edited a couple times before the whole menu system breaks again. Or maybe they are fine, until some pages get edited, then they become unresponsive again, or what.
If I switch themes AWAY from Pro, Menus are ZIPPY!
Page Loads are fast.
Literally hours of my life are spent waiting around for pages to load if Pro is active, or menus page to even respond to mouse clicks at all if Pro is active, or for Pro.