.iso install on IPMI very slow

I've been installing OS on my dedicated servers via IMPI and mounting an .iso file for a while now and while it's not lightening fast I can usually get a XenServer (or xcp-ng) install through in about 30 minutes. However in the past week is has been going really really slowly - what was taking 30 minutes is now taking around 10 hours! - I've tried from different locations including from a server inside OVH with the same results.

Any idea why it's running so slowly or even a better way to install the OS?