USB 3.0 is just fine, and MUCH better than Firwire, but not as fast as T'bolt. Unless you're doing huge projects with 2K+ files, USB 3.0 should be fine. Be sure that drive always has at least 10-15% of it's total capacity left as free space. Personally, I'm thinking your bottleneck is the GPU, and there's nothing you can do about that on a laptop.
How large is your Event/Project, and what format is the video media?
Mojo is known to render slow on any Mac.
What is the total duration of those 5 clips you're rendering?
If you have a 2012 MBP, it should be fairly good performance, with a USB 3.0 drive. But, did the 2012 MBP have USB 3.0 ports, or just USB 2.0? Cause if it is only USB 2.0, a USB 3.0 drive won't help. It'll work at the USB 2.0 speeds. I don't remember when Apple started using USB 3.0 ports on the MBP.
Here's what you do. If your USB drive is small enough, or put the clips on a USB stick, and go to the local Apple retail store. They'll have one with FCP X on it. Copy over the clips in to FCP X, do some experiments. They will let you do that. My students and clients do it all the time here locally in Baton Rouge and New Orleans when they're thinking about getting a new Mac.