Subtitle Package/Plug-in solution?

Hi - I've just subtitled a short movie using an opaque black bar and a basic title for hard-coded subs. However this has caused a massive increase in render time in the timeline and in export. I've looked at the captions option in FCPX but I can't change much in it fonts etc andit looks a bit basic - is there something I can do or use produce custom subs that won't increase the render time? Thanks!

Text slows renders down, that's simply a fact of life.

Captions have to adhere to the closed captioning standard, so you are limited to what those standards allow.