You can't completely turn them off. You can stop them moving by altering wind strength in environment options (main menu). To really improve frame rates use the replace assets tool & replace them with non speedtrees
That fix only stops them moving, it does not improve frame rates on low end systems, (i know already tried it). Only replacing speedtrees completely gives faster frame rates
Go into the route in surveyor.
Open the replace assets function.
Replace the sped-ed octopus with a superior, flat plane tree. (I reccoment Trunda's from the site below.)
The route will instantly look, and run better.
