Are you working with a ton of big layers? If you work like I do, you are probably making use of a lot of layer masks. Although I love layer masks, using them means that you are also requiring Gimp to keep a lot more data, some of which is not even on the canvas, and never will be.
For example, let's say you are using fur from a photo of a wolf. Most of this image (the head, eyes, ears, and the rest of the body in general) is not being used. Plus, the photo is probably a high-resolution photo may be rotated awkwardly, so that most of the image is not even on the canvas. If you do this 10-15 times, you will begin to notice that Gimp will start to crawl.
To fix this, you're going to want to do 2 things.
1.) make sure you have Gimp optimized for your computer by watching lesson 2 of my gimp for beginners series here: [link]
2.) occasionally click on the crop tool, and crop the image equal to your entire canvas. Doing this will have no effect on your final image, but it will cut out all of that extra "stuff" outside of your canvas that you're not using. I've had image files drop in size from over 200 MB to under 100MB just from doing this. You can almost hear your Gimp sigh in relief.
These two things will have a dramatic effect on how will Gimp runs on your computer, but you also may find that it is still running a bit sluggish even after doing all of the things above. If all else fails, there is a well-known bug in Gimp 2.8 where the color management can sometimes make Gimp come to a crawl. Try turning off the color management in the preferences window. More speed tips, check out lesson 2 in my gimp for beginners series.
Hope this helps you guys!