I have seen the same thing which I think you describe. In my case, I'm on an old Windows 10 PC. My power usage profile is set for power saving.
Have you noticed any dropouts? The manual says dropouts can occur if the LMMS CPU monitor goes into red. But I have not noticed any dropouts, so I am happy so far.
I have noticed that some VSTs take a long time to load on my PC. But that's not really a serious problem. Once they are loaded, everything is good.
Basically for me there are no problems running LMMS on my old PC.
Maybe if you provide more technical details about your setup and the exact conditions that occur, someone who knows the LMMS internals can explain?