Yea I have the same problem on some of my cars...but it shouldn't be a matter of speed, it should depend on RPM. Some of my cars won't boost(the gauge shows full boost, but the car doesn't want to move)untill over 4000RPM. Just check what engine you are useing,if it's NA it might not like the turbo, and if you put both NA and turbo on at the same time it will slow it down. I suggest taking the turbo off, going to the ini file, and changing the power multiplyer(line 440)to like 1300(power X 1.3) or as high as you want it. Then go into the game and buy the normal turbo for the car, usually you won't have a problem with it.