This is usually caused by network congestion. There are 2 methods for Miracast:
1. Wi-Fi Direct which is a direct connection between the Wi-Fi cards of the laptop and Surface Hub
2. InfraCast (Miracast over Infrastructure) where the stream is sent through the internal network.
Both can be affected by the network. The first one is mostly affected by the radio congestion in the area. You should do a site survey and see which channel is less used and configure the Surface Hub to use that specific channel. Being a wireless connection has it's limits so you might still have issues, especially if the laptop has to share the same Wi-Fi adapter between Miracast and network connection (access point).
InfraCast is usually affected like every other streaming application that requires a steady and large bandwidth. So if there's a bottleneck somewhere and the stream is not prioritized, you will have the same issue.
It's great that you opened a case on this issue as the engineer can check which connection is used and what needs to be changed for a great performance.