I've struggled with slow lineart for aaages. I'm still not really fast, but I recently started doing something that bumped up my speed quite a bit. Long, confident strokes with a full opacity brush. Avoid the urge to sketch or "paint" your lines. This probably seems obvious to lots of you, haha.
I've been watching Ursula Gray's videos on youtube (illustrator of the Magpie) and have been kind of studying how swiftly and confidently she creates her lines.
Confidence is really key. I've never trusted my lines. I've always been cautious, careful, meticulous. I've also rarely used a crisp brush at full opacity. I always go for kind of wispy, faded lines (I still like the look of it tbh) and apply them the same way I would approach digital painting. I enjoy digital painting, truly, but this approach was painfully slow and not at all fun for me. Working more confidently is a lot more satisfying.