I have Chrome users for browsing, personal email, work email and Slack. I had Slack in a separate Chrome window, in a pinned tab. I noticed that changing to my communications space was slow. By eliminating windows one-by-one, I discovered, the culprit was the Slack pinned tab. I tried the Slack standalone app instead, and that slowness is gone. I can't explain why, but the standalone native app may actually perform better. I think it's worth checking out.