The GIMP is optimized to speed i.e. speed is more important than memory usage. Enabling "Conservative Memory Usage" will "optimize" GIMP to use less memory. The tradeoff is of course speed but if your system is low on memory then it can be an option to use "Conservative Memory Usage".
By default GIMP has five levels of undo. The amount of undo levels is only limited by disk space. A high amount of undo levels requires a large amount of disk space so use it with care. The default value is reasonably good for most use.
This is the most important parameter that GIMP has. If you set it too low, GIMP will be very slow. There isn't a upper level of how much memory that you can give GIMP. That is to say, give GIMP as much as you can. But don't give GIMP more that your physical memory (i.e. pure memory without swap). In reality you have to measure how much memory you usually use and give GIMP a little less than the rest of it.
When you scale an image (make it bigger), you have to fill in missing pixels. The color and value that those pixels should have is calculated in a interpolation algorithm. GIMP uses a Linear interpolation algorithm by default. Linear gives you a mid-quality interpolation. Linear interpolation has less quality trade off for speed than Nearest Neighbour interpolation algorithm does. Nearest Neighbour is the fastest way to interpolate, but it has the lowest quality of the interpolation algorithms. The Cubic interpolation algorithm produces the highest quality of the three interpolation types supported by GIMP. However it is considerably slower than the linear interpolation algorithm.
By default when you save an image, a thumbnail file will be created — storing a little image of your image. When you mark an image in the file open dialog, you will see this thumbnail. If you set this option to Never, you have to press the Thumbnail button in the File Open Dialog to be able to see a thumbnail of your image.
This is by default set to "Always" but if you save your image on a networked disk (NFS, SMB etc.), saves are sometimes slow and saving isn't always that necessary. Instead you will only save when the file is really changed.