In my opinion, all 20” bikes will have similar rides and the difference between 406 and 451 is negotiable. I do start to notice a difference between 24” and 20”. Terns eclipse series has 26” wheels and they even have a sub $1,000 model new. If you do buy a used Tern, make sure it wasn’t included in the recall a few years ago.

For 20” wheels, I think the choice of tire has the most impact on speed. Kojacks or thinner will be decently fast, but the trade off is your ride will be more bumpy. The Mu Sl likely comes such with thinner racing tires.

The only 20” bike that rides differently is the xootr swift, which feels much stiffer in a good way since there is no hinge in the center tube. I also find the handlebars flex far less than Dahons which have a hinged joint. Would definitely go with swift if you can live with the bigger fold. It gets pretty small if you remove the handlepost and front tire. I believe there are the internet posts about converting them to 451, but you’d also need to get longer reach brakes too.