With Vulnerability Protection off, I'm getting 500+ Mb/s down, as measured by Speedtest.
When I turn the Vulnerability Protection on, my download speed drops to ~100 Mb/s.
Able to reproduce this 100% of the time.
Depending on what server you are using on your VPN (Virtual Private Network) it will slow down the speed. You might try testing different regions or countries to see how it affects you.
With me it slows things down by about 10% but I know of others that have reported significant slow downs like you have. There is a lot more work being done to encrypt and decrypt data along with movements along more servers that causes the slow down.