Welcome to the community, thanks for reaching out.
What you are experiencing is normal. Due to the technical nature of pairing your two speakers and the way Bluetooth data is processed, the Bluetooth audio data is first sent to the master speaker and then relayed to the grouped speakers after processing, Latency can occur when watching videos because this technology was never developed with video playback in mind. The delay is exacerbated because you're extending the chain. If you wish to also have your SoundTouch 10 grouped whilst you are referencing to video, a cabled connection would be required in order to remove the latency.