1. If this is an old build that has slowed down, when did it start?
What might have changed?
Software update?
Malware?
virus?
System restore to a previous good point can often fix such things.
2. It there a thermal issue?
Is your idle temperature 10-15c. over ambient?
If not check your cooler, it might have loosened, or the pump in an aio cooler might be restricted.
3. Run windows task manager.
Look for apps that are unknown to you or are using heavy resources.