I share your experience, somewhat, though I don't think I totally match your sense. It is worth noting that the hill profile is such that I do a lot more climbing on the ride home. I too ride MWF as a general rule, though I mix that up some.

Let's start: My commute is about 20 miles each way (it depends on my route choice; the shortest way is about 19 and the long is a little over 21). It is rolling terrain, and mostly MUP, but a fair amount of street cycling as well. While it is rolling, There are a few pretty steep, but not terribly long, climbs. (as an aside: the weather was gorgeous this morning; temp about 60 and sunny).

My average rolling speed (as determined by Strava) tends to be around 15.x (where x <5) in the morning and 14.x in the afternoon on my faster days. On my slower days, my morning speed drops to 14.x and my afternoon speed drops to 13.x. Interestingly, because I also track HR, my avg. HR in the morning is between 135 and 140, while in the afternoon, it is generally right around 150.

As to speed/ days: this is more complex: I definitely feel more willing to push harder on Mondays, but my average speeds don't necessarily seem to bear out any relationship. I know that the rare occasions when I ride two days in a row, my legs feel heavy on that second day, at least at the beginning of the ride in, and near the end of the ride home. But overall, my fast and slow days don't seem to relate much to where in the week it falls. Actually, I think a lot of it is psychology -- I ride in a place where there are lots of bike commuters, and I find I ride fast when there's someone who is just slightly faster than me to chase, or chasing me...