I was running into this problem. We were using phantomjs to run speed tests and weight tests for our web pages, and it all worked well on my machine.
Then it worked VERY slowly (five times slower!) on the build agent.
The fix was simple, once I single handedly googled it.
Go to Internet explorer on that machine.
- Click internet Options
- Click Connections
- Click Lan Settings
- UNCHECK “Automatically detect settings”
As an extra measure, you might want to disable IPV6 on the Ethernet adapter. Seems to fix it for some people.