My wife and I both have manuals and we sit in stop and go traffic every day. Having to shift in traffic doesn't even register with my brain that it's really anything at all. I simply don't notice.
It's a burden if you're stuck in bad traffic, if you've got a heavy clutch, or if you're crawling up a hill. Worse if you've got more than one of those compounding factors.
For me, motorcycles in traffic has been painful due to having to hold a heavy clutch in with your hand.
I really don't get what the issue is. Maybe my ADHD benefits focus-wise from having another thing to do? I rarely even use the cruise control unless I'm on a 3+ hour trip.
The same applies to pretty much everything in driving, of course. Initially, when learning to drive, the information flow overwhelms people, because the brain tries to understand and process everything. Eventually, routines take over and the low-level things like how to turn the steering wheel, or how to handle brakes or shifting, is done unconsciously.