Not always a lot of traffic. My travel schedules usually work out so that I am going westbound or southbound through KC on a Saturday or Sunday morning, traffic is reasonable then. I don't know about weekdays.
When going through, rather than around, be prepared for lowered speed limits, narrower lanes, and lane changes to be in the correct lane for an interchange. I-70, I-35, US-71, US-169 kind of get woven together downtown and then separate again.
I am usually going through on I-35, or at least departing on I-35 westbound, because I-70 is toll road to Topeka and I'm usually headed for Wichita, so on I-35 I don't hit the turnpike until Emporia.