Are you saying your Excel sometimes slows to a crawl when you use formulas? Because something similar has been happening to me as well and it’s terribly annoying.
No no
It has a bit of a lag or a latency related to the little helper popup / autocomplete formula thing. So if I press the tab key to accept the autocompleted formula suggestion, instead of Excel doing that, it actually closes the formula and moves to the next cell (because the formula suggestion autocomplete thing had in fact not finished popping up before I had pressed the tab key...). Whereas before that update, and now that I downgraded, its a seamless intuitive UX
Regarding Excel slowing to a crawl:
Chances are the workbook has become 'volatile', and at that point the workbook needs to be entirely recalculated on each change (whereas under normal circumstance Excel will 'know' that all the other calculations outside of the cell(s) affected by the change are remaining the same, and it will calculate only the affected cells). The easy solution is to turn calculation to 'manual' under the 'formulas' tab (but don't forget to recalculate then....). The ideal solution is to build the formulas in an efficient way for Excel...
Google around a bit on that maybe will find a solution.
I did notice that after downgrading to Excel 2013 last week I am seeing issues like that one more often, which I didn't have at all in the up to date versions....