Is there any image application that can crop images that is faster/lighter than GIMP?

To crop images with Shotwell (the default picture viewer) just open the image and choose Crop from the Photo -> Tools menu. This will give you a highlighted frame you can drag with your mouse.

You can use Shotwell which comes installed by default. just right click an image and select Open With from the context meny and then select Shotwell. Shotwell comes with a CROP feature so you can select which part of the image you want.

If its something you want to do more than once (say from multiple screenshots) then its best to Alt-Print Screen to perform the image cuts and then use Imagemagick convert as follows:

convert -crop 400x300+50+200 screenshot.png test.png

will generate a 400x300 image as a subset of the original screenshot.png offset by 50 pixels east and 200 pixels south.

Its slow to sort the first time, but once you have done it once the other images can be put into a batch using a bash script.