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.
These days we have Strava for this. Check for example (Strava account required to see details of individual rides):
You'll find some segments there, and quite often you also get tracks from pro cyclists. From looking at just a few KOMS I see that top speeds at about 110-120 km/h are not uncommon. You can find interviews with pro riders where they boast about speeds up to about 140-150 km/h. Above that seems highly unlikely because you're approaching terminal velocity and at those speed pedalling isn't going to help you.
Note: Updated to show actual segments (no account needed), but to see maximum speeds, you do need an account.