The installer tries to download the manifest file, and if this download does not work, then the mirror choice is restarted without this option. So if mirror is down, or it does not have the wanted version (e.g. was not synced in time), then the installer should be able to ignore it automatically and find a better source.
If the mirror gives away bad data, than the whole install will stop with error during "verification" phase. That's a bad situation which I hope won't happen. In any case, I hope such issues will be reported quickly, so that we could remove the failing mirror from config and see what's going on.
If the mirror is slow... that's very sad Note that currently when you install fresh 2.08 version, you have to download full 2.00 version and 8 differential packages, and each of these 9 packages is downloaded from independently chosen mirror. So if one mirror of three is 100 times slower than usual, then everyone will suffer by various degree.
The user can manually remove some mirrors. To achieve it, run the installer at least once to the version choice page, then exit. Open tdm_installer.ini and comment out the failing mirror from [MirrorSet TDM_MIRRORS]. Then restart installer and check the "Skip config file download" in advanced settings.