[KevinH]

Yes removing ids can in fact break external links into the epub (see the cfi spec used by most Readium based platforms and calibre as well). They can be used in toc's, pagelists, references for internal javascript, and of course references used for linking.

As a page is loaded, a hash table of ids to dom nodes is created faster than the io of the file on the disk can be done. Jumping to an id is basically instantaneous in epub readers unless the page itself is absolutely huge (entire book all in one file).

Please cite the exact speed test for loading a page you talked about. There would have to be an excessive number of ids and tons of images to load to make even a slight difference.