I've been using gmail mostly via IMAP for at least a decade and didn't notice any trouble. The only unusual things that I could notice is that gmail has labels, not folders, and some folders live in [Gmail], but otherwise I didn't see any serious trouble there.
Same here. I use Thunderbird via IMAP and gmail hasn't slowed down at all in the last 10 years since I've been using it. I don't use their webmail interface or gmail branded mobile app either. Email is email. Use an actual email client.
