we can't, for example, make a ride last 2 minutes if it's 3. But some flat rides have sequences where you can add or subtract sequences. Example, if a flat ride spins in both directions the sequence might be set to - Forward Spin - Forward Spin - Backwards Spin. You can add more Forwards or Backwards spin (or delete them) as many times as the ride allows making it shorter or longer.
Coasters: the only speed you can control is the chain lift. The speed, just like in real life, depends on the first drop. You can't set the speed to 60 mph or anything like that.