Spreadsheets are really slow in FreeCAD. I'd suggest you look into Varsets [0] if you don't need some of the fancier capabilities that spreadsheets provide.

[0] https://wiki.freecad.org/Std_VarSet/en

Yes, VarSets are definitely better (more granular dependency recalculations).

The little-known Configuration Tables aspect of Spreadsheets is absolutely worth using, though, and has no equivalent in the VarSets scheme at this point. Once you get that set up it is great fun.