I'm curious about this. I don't plan to do anything about it, but would like an opinion anyway.
I've been sewing on an old Touch and Sew 758 (even though I have a lot better machines stored and ready to go in the garage). When I first start to sew, it goes fairly fast. The longer I sew the slower it gets.
I usually don't sew for more than an hour or two at a time. I really noticed this yesterday. I was doing a lot of start and stop sewing as I was sewing blocks into rows. I finished the last row today and it was sewing fairly fast, not 201 fast, but faster than it was at quitting time yesterday.
Logically, it doesn't make sense. You would think it would go faster as it heated up.