The Pixel C is, by all accounts, a very fast tablet in general. But that may not be the case when it comes to the device's WiFi connection. There's a growing thread on the Google support forums of Pixel C owners complaining about sluggish WiFi. Many say they are seeing download speeds that are a mere fraction of what other devices get.
Based on the responses, this issue is most apparent when you start to move further from the router. As other devices maintain a consistent connection, the Pixel C starts to fall off drastically. Upload speeds are consistent with other devices, but downloads are less than half of what they ought to be. No one has found a setting tweak that makes any difference, either.
Googlers have responded in the thread (and via support requests) to indicate they are investigating, but this was in the middle of the holidays. If there is indeed a bug, we'll probably hear more now that everyone is back to work. For what it's worth, the Pixel C units we have access don't seem to be affected by this issue, but it might be dependent on the router used.