We had the exact same issue and found, ultimately, that it was a result of channel switching. As our Wi-Fi Access Points are set to auto-switch channel if it becomes saturated, each time the Surface Hub Miracast channel was changed to match the nearest AP, the AP would then switch to something else, causing poor performance.
See here for more info, specifically:
If Surface Hub and the projecting device are both connected to Wi-Fi but using different access points with different channels, this will force Surface Hub and the projecting device to channel switch while Miracast is connected. This will result in both poor wireless project and poor network performance over Wi-Fi. The channel switching will affect the performance of all wireless traffic, not just wireless projection.
Channel switching will also occur if the projecting device is connected to an Wi-Fi network using a different channel than the channel that Surface Hub uses for Miracast. So, a best practice is to set Surface Hub’s Miracast channel to the same channel as the most commonly used access point.
Ultimately, we have instead ensured that all our users are running at least Windows 10 version 1703 (Creators Update) which introduced Miracast over Infrastructure, and also ensured that the Surface Hub and user's device are on the same network (our corporate network) which thus allows the device to default to Miracast over Infrastructure.
