That sounds like it is probably a DNS related issue. Are your clients hard coded to a specific set of servers?
I've actually been noticing similar for the last week or so. I did some tests this evening.
SpeedTest over Sonic OpenVPN (IPv4 connection) to Sonic in San Jose:https://www.speedtest.net/result/14784314343
SpeedTest over AT&T to Sonic (IPv6) in San Jose:https://www.speedtest.net/result/14784332317
SpeedTest over AT&T to Pigs Can Fly (IPv6) in SF:https://www.speedtest.net/result/14784326545
SpeedTest over AT&T to Fastmetrics(IPv4) in SF:https://www.speedtest.net/result/14784364841
SpeedTest over Sonic OpenVPN (IPv4) to Sonic San Jose:https://www.speedtest.net/result/14784527489
I've been noticing my speeds over OpenVPN have been pitiful all week. The modem was reporting the correct line rates (user rate: 25311 kbs/2036 kbs and max: 49386 kbs/2036 kbs) and I was wondering if maybe something was throttling me, so I gave the above a test. I haven't tried to see about forcing an IPv4 test from AT&T to Sonic in San Jose, though I am not convinced that will make much of a difference. (Right now I force connect to the IPv4 OpenVPN interface.) It also seems that I wasn't able to try to test with any SpeedTest servers at Sonic in Santa Rosa (at least not easily from their Select a Server option, despite seeing listings from all around the globe).
With my current hardware I am used to getting about 11Mbps over OpenVPN (I'm in the process of upgrading my edge router). But recently it seems that it can be around 3Mbps, 5Mbps, or 7-8Mbps.