load testing through wifi/lan + vpn

I really did not found answer on my question in the web. I am currently doing load tests for a web service, for example: how service will handle 15 threads in 1 seconds, for that I use Jmeter. I always get different average response times for 15 threads. When I'm in my company's inner network, I get wonderful results, but when I am at home, using lan/wifi + vpn to get access to that web services, I get horrible results. When I test it through vpn, web service can not handle 30 threads in 1 seconds, average response time is like 13 seconds, otherwise from company's network, average response time is 4-5 seconds. Also, that web service, would be also called from a system using vpn. My question is, what is correct result and correct way to test it. Test it from company's network, or though vpn?