IMHO FSLTL on default is way too intensive, and more pertinently, unrealistic.
Try these settings And see how you get on;
What about your other settings? Cars, boats, airport vehicles etc?
Airport vehicles should be set to 1, Ground Aircraft and worker density should be set to 0
Road vehicles 20-30 and boats whatever fits best for the method you use.
Finally, set your TLOD to 205 and see what difference those settings make.