What are typical max speeds for pro downhill races? Looking videos like Red Bull Rampage or Mega Avalanche make me wonder about the speeds attained by those pro bikers. How fast they go in straight line sections?
The differences are quite significant from race to race - each track is different. Also events such as Rampage are not too much about speed. The top speed may vary between 55-65km/h on tracks such as Mount Sainte Anne up to around 80km/h in Pietermaritzburg which is known for high speeds achieved. Of course all assuming good weather.

Also these speeds are kept only for a few seconds (in the best case), the average speeds come around 30km/h mark course and weather dependent.