Teams is also an incredibly successful product, moving far beyond Slack in terms of usage, so it's an interesting case study. My guess is that speed just hasn't been one of the top priorities, and they have instead prioritized cramming lots of features in there, both to copy Slack's functionality and to add integrations into other Microsoft products that help make the sale when they're pitching Teams to customers.

It is certainly aesthetically displeasing for such a relatively simple product to be slow, but it often makes business sense that it's not a top priority.