Space saver spare tires are smaller wheels and tires that can be fitted in the event of a puncture. They are generally only rated to 50 mph, and short distances. This Quora answer sums up the impression I get about their performance characteristics:
Q: Why do spare tires have speed limits?
A: Because ALL tires have speed limits, the difference is the particular “emergency use only” tire has a significantly reduced capacity for load and heat dissipation and so the “limits” are “prominently displayed” so you don’t exceed them, destroy the tire and wreck your car”.
Why are they built this way? It is certainly possible to build smaller tires that can take the load and heat dissipation of high speed, motorcycle tires and old F1 cars are the obvious examples.
Having a different size wheel is obviously going to affect handling, but is there a mechanical reason for a space saver wheel/tire to be less able to handle the load and heat dissipation of 70 mph than a regular wheel/tire?