The problem may have coincided with 18.0.1 update? But something just happened which was not at all noticeable in recent versions.
I do a lot of work in Yahoo mail and suddenly working with drafts I notice sluggish keyboard response while I'm composing. Backspace is noticeably slow, and generally working in body of text sluggish. Even the text that I type seems at times to appear at the cursor sluggishly within Yahoo. I'm working in rich text of course.
Don't notice this behavior yet anywhere else. Drives me nuts.