Every increment of the mouse fires an interrupt, forcing the system to only use HPET raises the amount of interrupts processed per the tiniest movement of the mouse, you can actually measure this with WPR in interrupts being processed at a higher rate, dpc can also increase as a result. In the case of high cpu usage and especially the high creation and destruction of threads it can make the mouse not responsive or not react with any precision at all. The "mouse smoothing" reported by people who force the platform clock are actually suffering higher latency / buffered input, rather than experiencing better mouse performance and those sensitive to mouse floating will detect it easily in the conditions it comes about. I fell victim to the "use platformclock" for better gaming too back in the noughties and games that tax the cpu harder are most likely to demonstrate this. Mouse float that people typically blame on their frametimes or buffered frames.