In short, trains slow down in 90+ degree weather as a safety precaution because extreme heat can cause the rails (which are made of steel) and the overhead power wires (which are made of copper) to expand.
Like in other cities, our system is designed for the average temperature ranges of our local climate. When temperatures are at the extremes of that range, the materials in the system are sometimes unable to adapt.
Read more about the effects of extreme heat on the transit system