Comparisons like this are useful. I’m on a similar system to you - Pop OS 20.04, Intel i7, 12 GiB RAM, and Mailspring never gets up to large CPU consumption. I see two differences between our situations:
- You have a Gmail account, whereas I do not. (I have a dozen email accounts, though!)
Does Gmail demand more CPU somehow?(EDIT: I just linked my old, message-filled Gmail, still not using much CPU.) - I don’t have very many large threads. Most of my emails are <= 4 messages.
This is an ongoing related bug, which may in fact be what you’re experiencing: Significant Lag on Large Threads