I'm writing on Saturday a little before noon, when the problem is usually much less pronounced. I just tested with SpeedTest, and got:
Without VPN: down 224 Mbps, up 272
With VPN: down 140 Mbps, up 38 Mbps
I also did an "eyeball test" by visiting two similar jam-packed Web pages, and could see the much slower download with VPN active, so this doesn't seem to be a testing anomaly.
During high-traffic periods (weekday late afternoons), I can see speeds as low as 30 Mbps ↓ and 20 Mbps ↑ with the VPN active.
I'm particularly concerned about the much-slower upload speeds, because where I am most concerned about VPN privacy is when uploading files. Is Sonic planning to upgrade the speed of its VPN servers/service?