Important question about adding theme

[Studio1one]

As I'm not a super pro when it comes to coding etc but is it possible to add a theme and still have all coding, speed etc as it is compare to using the classic original theme?

I started to set up a 1.7.4.2 and was able to fix 98% of everything that was an issue at the code, we tested it in Gtmetrix, seositecheckup, pingdom, screamingfrog, W3C etc.

Everything was fantastic, the loading speed was 0,5-0,6sec, then we added a LeoTheme and all went crazy.

Is it even possible to remain same as it was in classic theme?

If not it means that to get a 100% issue free store is impossible when adding a external theme. :-(