MS Excel extremely slow on MBP with M1

What version of Office/Excel? That determines if it uses Rosetta.

Something else to try. As MS Office/Excell is not an Apple product and there are not as many Office users active here as one would think, I believe you will be better served by asking in Microsoft's Office forums. MS Office questions posted in Apple’s forums tend to linger a long time before getting any kind of helpful response.

The only trick is limiting views to Mac issues, and that’s easy.

Start here: 

Results in Office - Microsoft Community.

That will display this page header:

1) If needed, use the “Office Topic” pull-down to select your Office app.

2) Limit answers to Mac topics with the “Office Sub-topic” pull-down.

3) If needed, use the “Show Filters” option to further narrow the results.

Everyone there is both an Office user and a Mac user, something you can't say about Apple's forums. I believe you will find the contributors there quite knowledgeable and very helpful.

Hey Bob Basket,

It sounds like you're having an issue with the Microsoft Excel app running slowly on your Mac. We can understand wanting to look into that further. We want to help.

We'd recommend trying the steps in this article: If an app on your Mac stops responding, closes unexpectedly, or won’t open

Thanks!