Recently simulators have not been able to be downloaded from Xcode, after endlessly trying the transfer rate was well below 100kb/s and would fail after a short while. The speed was on average 100x slower than other servers with the exception of a short spike of passable speeds.
Because Xcode for some reason doesn't support resuming simulator downloads, I downloaded the simulators directly from the dvtdownloadableindex file using two different methods but the fileSize values consistently differed slightly from the downloaded files.
This resulted in a Missing Signature error when adding the simulator with xcrun simctl runtime add.