Generally, if the machine is moving back and forth like a dot matrix printer, it is because the image is being Engraved instead of Cut. The machine moves a different way for each function. (And yes, engraving takes a lot longer for large images.)
There is a beginner’s guide linked below that explains the different kinds of input that you are sending to the machine, and how the machine treats each one. It’s a good one to start with to learn how the laser is going to act when it encounters different kinds of input.
Laser Design Basics
After you read through that, there are other tutorials that explain a little more about the differences between raster and vector input, and what happens with them. (Cutting requires vector input from a program like Illustrator or Inkscape.)
If you haven’t run through the starter prints that Glowforge provides yet…those are designed to let you practice with the different kinds of cuts and engraves, so it’s a good idea to try them out.
The third one (the Gift Tag) gets you started creating vector cut lines in Inkscape.
Make a Gift Tag from Scratch