You can assess for yourself the tradeoff of image degradation as it is further compressed in the examples below. Not every image should be compressed to the fullest degree possible, but the difference is not that much, especially if you are not comparing images side by side, and generally speaking, load time is much more important than unnecessarily high resolution. Progressive display of images is slower than regular downloading.

Download times are at 33.6 k.
29,703--8.3 seconds
20,361--5.8 seconds
7414--2.4 seconds
15,100--4.6 seconds
13,710--4.2 seconds
5060--1.5 seconds
9090--2.8 seconds
8360--2.5 seconds
3270--1.0 seconds
6345--2 seconds 
5950--1.8 seconds
2480--0.8 seconds