Network response times faster over VPN than straight internet

I have a monitoring station that does a simple web page check on a bunch of web servers at several sites, just checking to see if they are up. Around 12/20/18 the response times increased by a factor of 10 and in some cases 20+. I set up additional sensors to check the same sites via VPN and the response times are normal, a fraction of the time it takes over the internet. From what I've read the usual explanation for VPN being faster is that your ISP is throttling you, but it's just a sad little 1.5Mb T1 so I can't imagine it's being throttled.

I set up additional sensors to check over the internet by IP rather than DNS but they are just as slow.

I set up a second monitoring server about 1000 miles away and it returns normal response times, much faster than the monitoring station which is relatively local to the remote sites.

Trace route from the monitoring site to the remote sites doesn't showing anything unusual. I guess I need to call the ISP but they're generally pretty useless for anything other than basic stuff, so I thought I'd ask the folks here if you have any ideas what might be going on, or what other tests I could try.