How vulnerable is your car to loosing power when the temperature increases?

I noticed a massive difference over a weekend on the same motorway when the temperature dropped from 26C to 18C.

When using cruise set at 70, the car would shift down to 5th on the hot day, but spend more time in top on the 17C day and feel significantly quicker. It was very noticeable indeed, car was fully loaded, same route, same fuel etc, so it was directly comparable.

It spent so much time in 5th (and even 4th) on the hotter day that the engine temp increased, prompting me the set the cruise speed at 60 to allow it to cool down.