Recently at more than one site we've been experiencing slowness with file transfers, and general 2012 behaviour. It's maddening because task manager, the performance monitor and XenCentre show very little to no load across the servers. Having reviewed it more thoroughly and turned off Windows file security and made no progress, we've started looking into the hardware that makes up our XenServers.
It's a bit of a mis-mash of gear - an IBM x3650 and a generic sort of a server make up the two physical hosts. They don't have a huge amount of power under the hood, but run a couple of VMs quite well. The 2012 server runs appalling though and I think I've figure it out.
The x3650 has a Broadcom chipset on the network cards and this doesn't play well with others. The other generic beast of a machine has an Intel chipset on it's network cards and it runs fine. Yesterday I installed an Intel network adaptor into the x3650 and lo and behold, it's running better than it has been - significantly better. A click on the start button could take 20 - 30 seconds to get the menu to pop up in 2012 server, now it's almost instantaneous. My guys using this server for stuff are much happier.
In earlier iterations of XenServer I haven't noticed this so much, but in this most recent one I certainly have. XenServer 6.2 didn't seem to have this issue, so I wonder what has changed in the driver management to have caused this issue.
If you have servers running with odd slowness, definitely check this out - we have a much larger site with 2012 servers running on hardware with Broadcom chipsets and we are about to install new Intel cards to see if that fixes the problem. Stay tuned - this could be a real head scratcher if you come across it, and if the fix is a couple of $500 NICs then it could save you a huge amount of time and effort.