If coded as per the recommended standards, every theme is highly performant.
But this is not the case with most third-party themes out there.
Most third-party themes come with a lot of customization options and code bloat because they try to satisfy the design needs of 100s of business owners.
And this results in a slow website.
So, if you care about the performance, choose your theme carefully.
If you have the budget, go for a custom theme because it gives you ultimate control over the design and performance of the website.
If you don’t have the budget, here are the most performant third-party themes according to my experience with client websites:
Yep, there are not many.
A few good things about these themes are: