It would appear to me that all questions tagged as performance-tuning are actually regarding performance related. However, the same is not entirely true for the reverse (hence the order I've suggested.)
A question regarding tuning the performance of the SE network is still valid as "performance", but a question regarding the performance of SE is not valid as "performance-tuning". I know this is fixing something that's not broken, by why have more tags (hence being harder to follow / sort) than need be? Especially when the meaning is synonymous.