Still, there can definitely be a perceived speed increase with some image slicing. Browsers do benefit from parallel downloading, but too much slicing is worse than not enough. Some images do have features such as repeating elements or areas with different compression styles where slicing can help. In some sites, people may also perceive a page as being faster when the total download is actually longer. It's hard to make firm rules about the effect, but it can be useful to test a new design with a few slicing schemes.
Differences are easier to see on an older machine, with a modem and a dialup connection.
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