No or poor connection
The amount of data transferred is dependent on the services or apps in use in the car. For example, streaming audio can require large amounts of data which requires a good connection and good signal strength.
Mobile phone to car
The speed of the connection may vary depending on the location of the mobile phone in the car. Move the mobile phone handset closer to the centre display in order to increase the signal strength. Ensure that there is no source of interference in between.
Mobile phone to network
The speed of the mobile network varies depending on the coverage in the present location. Poor network coverage may occur, for example in tunnels, in mountainous country, in deep valleys or indoors. The speed also depends on the agreement you have with your network.