I did the slow build and it took me 6 months to get the tail and wings done. Getting the fuselage to the quickbuild stage took me about a year and a half.
And I was putting in plenty of hours every week! So to answer your question, the biggest time saving is definitely getting the quickbuild fuselage.
As others have mentioned here in the forums, some of the quickbuild fuel tanks have been problematic with leaks and paint bubbling. I'm glad I was able to be super meticulous when I built the fuel tanks and so far have not had any issues (don't skimp on the proseal). You can also upgrade the fuel caps and senders by doing the slow build.
The canopy was definitely the hardest part of the build, just because of the ever changing nature of the parts and having to trim everything repeatedly to get a good enough fit. The fiberglassing part isn't bad, but it is very iterative - apply, sand, fill, repeat. Once the canopy is done, it is all downhill and tailwinds to the end.