Ask HN: Is anyone building an Excel alternative for Mac?

It already exists https://www.openoffice.org/ or https://www.google.com/sheets/about/

This is the default on Ubuntu 20.04 https://www.libreoffice.org/ (works with mac as well)

I'm using Excel from Office 365, and I'm not seeing "sluggishness" on mac (on an ancient 2009 iMac, 10.13 High Sierra). Not sure what version you're using.

I'm not a heavy spreadsheet user and I tend to use cross-platform applications but, quite frankly, Numbers works pretty well.