We had a bunch of slowdowns on iPhones here. Dropping to 600KBps on the network rebooting the comcast router seems like the main cure.
But macs have suddenly become unreliable.
I've been battling vpn software for a while. Used tunnelblick for a while but it seems unstable. Moved to privateinternetaccess'a own vpn but looking at console this seems to be crashing. Also tried dnscrypt but having trouble with DNS queries.
Whew this is debug nightmare. Uninstalling all this stuff to see if I can get to stability. When i uninstall things seem better. Getting 100MBps downloads! And no more errors. So I guess I'll add things in one at a time and watch the console
But I see lots of messages in the console about discoveryd and a google leads to Arstechnica and furbo reporting bugs that sounds so familiar. Basically what's happened is that Apple rewrote their mDNSresponder into a new process called discoveryd and that thing in Yosemite has lots of bugs that I've seen over the last few months:

• Random failures to resolve DNS Names. This looks like the browser has completely slowed down and it turns out that many command line utilities like dig don't use discoveryd, so you can't find it at the command line
• Duplicate machine names. I've seen this a few times where there look like Home Theater becomes Home Theater (2) for no apparent reason. Bugs are the reason 🙂
• Wake on Demand breaks. I've noticed this too, so Mac's don't wake up for things like our file server. Ugh.
• No more wide area services. I've noticed this too, I used to be able to login over the Internet to various machines, but discoveryd doesn't support any of the mDNSresponder protocols for doing this.
• Lots of console messages that look like Basic sockets could not setup as reported in http://apple.stackexchange.com/questions/165013/console-log-flooded-with-discoveryd-errors-how-can-i-make-them-stop-not-just-h where it notes you can diagnose trying nslookup which uses discoveryd and comparing results with dig which does not.

Overall not a good list of things. The scary fix is to use 10.9's old mDNSResponder. Heck, it does seem like I'm starting to live in the old world. Using iPhoto instead of Photos and now having to regress old machines to mDNSresponder.
To diagnose this, you can see slowness in the browser or try looking at namebench which is an open source tool that looks at thousands of DNS Servers to see which one is fastest. The result is that 8.8.4.4 the Google DNS is the fastest, but I think it might be an issue with the Comcast DNS as well because our Windows machines are also slow, so I've switched to Opennicproject for now and will report later.
I think I can also report now that using discoveryd with VPNs and dnscrypt is probably an unhealthy thing. The simple fix is restarting discoveryd thanks to masugadesign.com:
 sudo launchctl unload -w /System/Library/LaunchDaemons/com.apple.discoveryd.plist sudo launchctl load -w /System/Library/LaunchDaemons/com.apple.discoveryd.plist 
Finally I was pulling my hair out. Looks like the modem would just start and stop. That is suddenly we wouldn't see the internet and no pings would work then it would happen again. I reset it a few times. Finally called Comcast and she "reset" the modem from her side and it is fine. Just a sign that sometimes this technical support isn't obvious at all!
Things remain intermittent though, so hard to diagnose. Time to call Comcast again!