A large number of services can be set to manual so they start when you initiate them. A handful of others can be disabled, but would require you to manually start them from services if they need to come online to support a program. In general, you can read the description in services for each as a guide.

The core parking, GPU driver, etc. advice above is also very valid to check. LatencyMon and your Resource Monitor are also good guides for what the machine is doing.