[pwg]

Ten times slower than what?

A relative statement, without an indication of what you are measuring against, is of no value.

[sushidev]

Ten times slower than other types of traffic such as tcp

[pwg]

At the same time?

Then that does sound like your ISP may be de-prioritizing UDP data, or specifically throttling UDP flows to some level 10x smaller than your full bandwidth.

Are you sure, however, that the other end of the VPN connection is not where the slowdown is happening? If that end was the choke-point, you'd still see slower traffic but your ISP would not be the reason.