Maximum capacity before workbook becomes unstable

This is a general question to ask if there is any general size restriction on files or number of formulas or something like that that would lead to my situation as follows:

I have taken one workbook I made called "Main1" (only about 6mb, 10 worksheets, alot of data and macros though) and made a copy called "Main2". In Main2 I have added a small macro (just opens a workbook and copies in data) and changed some excel formulas to add in a few IFs and VLOOKUPs.

Now, Main2 lags so much during any task, even basic moving around the workbook or running the smaller macros. Main1 still works fine. What sort of things could have caused this given I haven't changed much and certainly have only made pretty vanilla changes. (Note Main 2 is only 6.05MB, same number of sheets)