That would be mostly your phone, Richard.
The Beeline is (almost) merely a "repeater" screen of the navigation data that the app calculates and outputs.
I bought the Beeline specifically to use it when in the city (so not to have to take the larger XT with me). I found that it works very well.
I have disconnections in some crowded areas between the iPhone and the Beeline, not GPS/navigation drops, but that is known issue I have with my iPhone 13 where in crowded areas Bluetooth connection is unreliable. Having phone conversations using my Airpods is a nightmare, as they disconnect every few seconds during the call, but this is an Apple-side problem, not Beeline.
Long story short: try using the Beeline with another phone while in the city (if I remember well, you use a dedicated device to pair with gizmos) and you most probably will have a better result. If it's slow, it's the phone connected to it being slow.