Where are you riding? I ride a fatbike at -20 degrees on snow. Works great, considering that there is no other practical riding solution.
On pavement, a hardtail mountain bike is 3mph slower than a road bike. A fatbike will be 6mph slower than a road bike. This will be for the same level of effort.
These differences are actually huge, as the power output required to maintain a certain speed goes exponential.