Regarding US 14 east of Janesville, I suspect the new interchange design at I-39/90 and I-43 is in part to encourage traffic to slide down to Beloit to get to I-43 rather than using US 14. The relevant ramps are very generous in their geometry. Once that's all done, I'd be interested in seeing the difference in travel time between any point in Janesville west of the interstate and exit 15 on I-43. I suspect it'll be close to a wash.
There's also the whole Interstate-to-Interstate interchange thing, or at least that's what WisDOT cited when they designed the I-39/90 / I-43 interchange. Basically, for safety reasons they designed it to be taken at highway speeds.
You are probably correct about travel times being negligible following US-14 east of Janesville v.s. I-39/90 to I-43, at least the way Google Maps calculates it. But it's an additional 8 miles, and even after construction is over most locals (including me) headed east to catch I-43 would rather take US-14 because it's more predictable, unless there's inclement weather (US-14 drifts bad between Hwy 89 and Janesville).
WisDOT has resisted adding passing lanes to US-14 in either direction from Janesville. It's not exactly a simple task; the 4-mile 2-lane stretch between County O and Hwy 140 has a lot of residences and farms right up close to the roadway, plus it goes through Emerald Grove. The passing lanes would have to go in just east of the Hwy 140 intersection to make any sense, but that's after considerable traffic turns off at Hwy 140, so it's less effective.
So, yeah, I see WisDOT's thinking in encouraging traffic to stay on the Interstates. But being a local who knows the area pretty well, I'm probably going to stick to the County roads to get over to I-43.