I have the same problem in Michigan with 24Mbps service. Been going on for a couple of weeks now.
Oddly, if I reboot the gateway, it'll go back up to about 20Mbps down. Then, over the course of 4-6 hours it'll slowly drop back down to about 1-2Mbps.
Even more strange, regardless of what the download speed is, I always get 2.5Mbps (give or take) upload.
Maybe it's the RG?