FSX Micro-stutters after converting Ground Poly


i converted some Ground Poly files into the fsx .bgl format, because the GP wasn´t visible in the sim before the conversion. But now my fps are sometimes low by around 10 fps for a moment and after that they are on a normal level. This is causing Micro-stutters in my sim. Is there a way to fix this? (Maybe i did a mistake with the MCX)