How much longer? I guess I could just wait while doing other tasks.
That would depend on which computers you are comparing, and what you specifically do. You can try to figure out the differences by reading some reviews with benchmarks.
Here's one example.
21.5-inch iMac (Late 2013) Review
You can see in the charts, that exporting the same project in iMovie '11 takes roughly twelve minutes on a 2008 MBP, while it only takes less than five minutes on more current MBPs or the 21.5" iMac. The actual times depend on the size and complexity of the project you are exporting, but you get the basic idea. A faster computer is... faster.
The only way to figure out if this difference matters to you, is to try it out. Try editing videos with your current laptop, and see if you are pleased with the speed.