I guess over the last 26 years or so, I've replaced hundreds and hundreds of zippers. This includes zippers on slacks, tops, skirts, jackets, wedding gowns, bridesmaid dresses, Halloween costumes and even a pair of boots!
The most time consuming thing about any alterations is the ripping. And once the ripping is done, picking out the loose threads and cleaning up the area will take even more of your time. This is a sign of a quality job, though. Don't cut this process short.
I can share a little trick with you today. When you rip out a zipper, you don't necessarily have to rip each stitch with your seam ripper. If the fabric around the zipper is stable enough, you can literally rip the zipper right off the garment. Of course you want to test a small area first, and don't pull too hard if the fabric won't give. But chances are, this will work for you and save you a lot of time.
This is a pullover jacket made of sweatshirt jersey fabric.
Picking out the threads is a different story, though. I haven't found a shortcut for this tedious task, other than doing it while you are watching a movie or talking on the phone. After all, it is a pretty mindless task.
So go rip out a zipper. Clean up the threads and get ready to put in a new one!