Hello Mark,
yes, Designer installed from MS Store is very very slow. Designer installed form affinity-download ist fast. But I bought Designer from MS Store and so I coult user Designer from affinity-download only for a limited time as a try-version.
Other programs from the MS Store run normaly.
I don't know how many fonts in my windows-installation are. But there are the some fonts when running Designer from the MS Store (extremly slow) and form affinity-download (in fast speed). I also use Affinity Photo and Affinity Publisher - both directly from Affinity bought. The are both running fast.
So I think it has nothing to do with ithe fonts. Designer ist extremly slow also when I habe opened no document.
BG
ArWi