Should I use all these template files or will theme hooks do the job? Since templates are slower than theme hooks, I'm wondering if I might be able to use them for my application. Here's what I'm trying to do:

Right now I have about 10 content types on my site and I've created a separate template file for each because I want each to have a &lt;div&gt; with a specific 'id' so I can target those pages with CSS. I also give each a separate template file so I can determine on a case by if they should have a secondary sidebar or not.

Those are the only reasons I'm using all these template files.

Two Part Question:

Can what I'm trying to do be reasonably accomplished with theme hooks, and if so, how?
The module that defines a template defines if it is a template or a theme functions, that's not something you have any influence over.

And yes, theme functions are faster, but they're also deprecated and will be removed completely in Drupal 9 (Because twig is far easier to write than PHP). With render caching and so on, the performance difference is no longer as relevant as it used to be.

The performance difference is between having a template or not. There's anyway a template, just not a custom one, and the only performance difference then is the additioal complexity that your template adds and what you are doing sounds trivial.

There might be reason to do what you're doing differently, you could use preprocess, but you'd need to at least share your templates/code (you should always do this when asking questiosn) so you can get a specific code example. But not because of performance, a more valid reason would be code/template management, it will be more work to make further changes if you have 10 different templates or just one.