Is it possible to compile single topics in a project?

I think it would be really hard to do, as there are a bunch of behind the scenes files that need to be updated in the context of the entire project (search, index, etc). If you "just" updated for the single file, it might have flow on effects on other topics, breaking your search for example.

So I think the entire project has to be compiled to update all these files correctly. But as Captiv8r says, you can set up publishing to only publish the changed files (I think it's this way by default), which can save some time.

To what end? Do you mean to generate a single topic and update a published project with just that topic? If so, that's the purview of Publishing and what publishing does.