I worked as a repair tech for years, and I found that many performance problems could be fixed with that sysinternals startup tool (I forgot the name, it's been a long time) and just disabling stuff. This would be 5 minutes if you knew what you were doing.
The second was hardware issues, especially slow hard drives that started to fail but hadn't quite failed yet. There was a tool to read all data and print drops in performance (again, I forgot the name). This was before SSDs became commonplace – I don't know what it's like with SSDs.
I was as productive as anyone in terms of number of customers helped/week, but looking at my coworkers there were definitely two types of techs: the ones like me who knew what they were doing, and the ones who didn't. I know this sounds rather arrogant and pompous, but it's true.
As a company, you can give people the time and space to develop this know-how, or you can't, which is how you end up with "just reinstall shrug" type of repair shops. I was lucky enough to work at a small independent shop where we were given lots of free space and autonomy, and some of us took this to become pretty good at fixing stuff in short turnaround times, which benefited everyone.
We developed a bunch of internal tooling for this, too. I maintained Anduril, our FreeBSD-based live disk with tons of stuff (the company name was Aragorn, so this seemed like a good name).
Of course, this store no longer exists. Or rather, they pivoted to business support, because there's just no competing against the online stores with €10 profit margins on every laptop they sell, and there just aren't enough consumers to care about quality support to make the financials work out.