What is the recommended solution if a computer performs slowly?

Not sure what the class/context is, but here's what I'd do to diagnose a problem like that:

- Check to see what programs have high utilization of system resources in task manager and end them

- Prevent background startup processes from occuring on restart or from running in the background during normal use

- Figure out what hardware component is bottlenecking the system and upgrade it (HDD to SSD would be one example)

- If the issue is malware related, I would "flatten" the PC by wiping the entire system and reinstalling the OS (though some malware can persist even across this)

Source: I've worked as labs support