Use the Number of Colors slider to reduce the colors to give you a good trade off between image quality and file byte size / download time. Generally the smaller the number of colors the smaller the image. Good values to try are 2, 4, 8, 16, 32, 64, 128 and 256 as these are at thresholds in the GIF compression algorithm. For some images you will also need to experiment with different values of dithering (see below) and perhaps smoothing.
Note: The Optimized file size is shown at the top of the Optimized image window. The download time and other information is shown in the status bar at the bottom of your screen.
Step 3 (GIF/PNG): add dithering