The Pi B+ doesn’t actually have Gigabit ethernet, it has the same old 100 Mbps RJ45 ethernet port all the Pi’s have, it supports “up to” gigabit ethernet if you use a gigabit USB to ethernet adapter (for example Linksys), however most people cap out at around 200 Mbps even with the best adapters because you’re still using USB 2.0 which doesn’t have the gigabit providing capacity of USB 3+…
You’ve also now got another adapter in the chain adding latency etc, which for DNS resolution and DHCP stuff matters.