Apparently FSUIPC and SU12 have some problems that cause shuddering. This was a suggestion from Fenix:
Just an idea to try if you have trouble.So far all seems good with SU12! One thing to note, there is a bug with FSUIPC in SU12 that causes it to call variables that don't exist, dozens of times a second - this results in stuttering/lagging whilst flying several types of aircraft. If you're experiencing stuttering after updating to SU12 and run FSUIPC we strongly recommend you update to version 7.3.18, followed by adding "NumberOfPumps=0" to the [General] section of your FSUIPC7.ini file. For more information and help, please contact FSUIPC as this is not a Fenix issue. Further to this, the newest version (7.3.19) doesn't automatically resolve the issue, so you may still need to edit the FSUIPC.ini file
