This is an area where the internet caused a minor revolution.
Pre-internet, the professional tuners like Bowkett, Venom, Airmasters and many others ruled the roost. They were usually a bit vague about the processes they used to improve performance. Sometimes, it was a simple de-burr, polish and lube. Sometimes, much more.
The non-pro guys tended to do what they did on their own in sheds. And only tell their mates, in the pub. Some had it right, some not.
The internet meant that the “pub” group in which the shed guys were chatting became a potential global audience, for free.
So we have moved from individuals with no audience bar a metaphorical saloon bar, and businesses with an interest in trade secrets controlling information to an environment when a bloke in a shed can discover and then globally share his insights for free.
As others have said, if you have the basic tools and knowledge (and time and space) you can do a 90% effective tune. The extra 10% remains challenging, requiring specialist kit, or savvy, or experience.
And if you don’t have the tools, knowledge etc, you are at best doing a service, even if you spend £70 on the replacement bits. Though that service/tune may still improve performance.
Assuming you start with something decent, the more you invest in money, tools, and engineering skill, the less you get out over time. But, assuming you invest them wisely, you still get something better.
(it's not a "proper" Porsche, neither).