Premature optimization is the root of all evil (or at least most of it) in programming
When you make an application, it makes sense to want to ensure it's fast.
No-one wants to sit on your website pages watching a loading page inch along.
So it's a good idea to plan in performance testing (perf testing) and ensure you can handle an increased amount of users right?
Sure! Perf testing is vitally important... but you can do it too early.
If your small side project is still buggy, doesn't work fully yet and is still in development. It probably isn't the time to performance test.
But why? You may end up locking yourself into weird or un-intuitive design decisions because you think it's necessary for your app to be fast.
Normally what happens in the software developemnt lifecycle, is developers: make it work, make it right, make it fast (attributed to Kent Beck).
Meaning developers: make the application work first. Then they add comments, clean up any weird hacks they did, ensure it's consistent and fix any code smells or issues they see second. Then only thirdly, make it fast.
This is generally agreed by most developers to be the best way to develop software (there are always exceptions!)