I bought the PLD Star Class today and I've found the stuttering to be unbelievably bad with it. No other steam locos run like it, and my PC is pretty overpowered for TS anyway. I haven't tried the performance mode but I'm not convinced that would fix it since it seems to be a glitch. I'm going to send a support email to PLD to ask for help, but if anyone has any experience with it I'd love to hear if there's anything that sorted it. EDIT: I've been checking some other steam locos and other PLD locos have the same issues. I don't remember them doing that in the past, which is weird. I did check performance mode with the Star and it didn't make a difference at all.
I figured out the issue, for some reason TS stutters when the audio output is set to 32-bit. Steam trains were the worst but from testing it affected other intense trains as well.
It's in the Windows sound settings. Usually it wouldn't be set as high as I had it set, so for most people it will be fine. I have my DAC output set to 24-bit 96000Hz now and TS works fine.