How to open a file with openpyxl?

Question

Python

In general, you need to work with .xlsm file through Python. Googling realized that I need openpyxl.
But the problem is the simplest, I try to open the file, but it opens terribly slowly (10 minutes). At the same time, if you open it in readonly mode, then everything works fine and quickly. Maybe it's because the file is .xlsm and not .xls? If so, then tell me which is better to use. I would be grateful for help
wb = openpyxl.load_workbook(file, keep_vba=True)