We had this initial slow speed issue too, as soon as you bypass the cloud filter its fine.
Got a ticket open too but it's stumped them as well.
Doesn't seem to be much rhyme or reason to what device or user it happens to though, other than if it happens to you, it happens every day just with some variation to length of time.
We have had success in disabling Ipv6 on the network adaptor of an affected device though, we're just working on a script to turn off Ipv6 across all our devices.
Very odd issue.