Overall just low performance from boot up or does the pc perform as usual and then suddenly decline?
Use Task Manager and Resource Monitor to observe pc's performance.
Check Task Manager's Startup Tab for something unknown being launched at startup. Or perhaps later on.
Task Scheduler may be involved. Could be something has slipped in there and is launching.
Process Monitor and Process Explorer may help as well.
Look for some app, process, or service running in the background. Determine what resources are being maxed out.