All cars benefit from slower speeds because wind resistance at freeway speeds really kills efficiency. However in stop and go traffic ICE cars more than offset this benefit by throwing away energy every time they step on the brake. On the other hand EV, hybrids and PHEV can reclaim some (but not all) energy during braking, thus only slightly offsetting the benefit of the slower speed. So in most cases you will get better mileage in stop and go traffic compared to freeway speeds. Many people wrongly believe that it is regenerative braking itself that makes stop and go traffic efficient, but that is not true, if it were then if you are driving on a 45 mph road with no other cars you should constantly step on the brakes (or paddle) and speed up again. No that would be less efficient because again regen does not recover all of the energy loss of braking. It is the lower average speed in stop and go traffic that makes it efficient compared to high speed.
Of course you will find out for sure by doing both routes to see which if any saves gas/electricity and whether it is worth the extra time or aggravation.