Reduced image size with no apparent reduction

[robin]

I have a page with several large images. Reducing the size by more than half produces no reduction in time reported load the image. What could I be doing wrong?

[pmeenan]

Any chance you have links to before/after tests? Make sure you run enough tests to get a consistent result (3+) but then looking at the waterfall will tell for sure, particularly if the images are not in the critical path.

[p83822]

Remember that download time is not linearly related to file size. What you are interested in is the number of tcp round trips - which will, of course, depend where the requests are in the tcp slow start cycle.

As a for instance, I have a 120kB background image which last time I measured it came down in the following numbers of packets on a new connection:

3, 5, 8, 12, 21, 37

i.e. 6 round trips. To get it down to 5 round trips i.e. only a 1/6 improvement, I would have to reduce the size by 37/88.