Since I don’t know exactly how the wearable app handles the entire customization process, this is pure speculation on my part:
The wearable app or the Wear app has always had major issues with handling watchfaces with lots of graphics. From what I gather, one of the issues is the connection itself.
Bluetooth is the default connection between your watch and your smartphone.
Let’s say you have 4 MB of graphics in your watch face. Every time you try to customize the watch face with the wearable app, the app tries to load all relevant assets to the phone (or where do they get the graphics from?). With Bluetooth 5.3 (Watch 6) you have a transfer rate of 1.3 MBps at best (the official value is 2, but this speed is never reached). This means that the transfer of 4 MB theoretically takes 24 seconds. Maybe not the original files are transferred, but only low quality copies to reduce the filesize. I have no idea.
When I turn on WiFi and KEEP it active! the wearable app is more stable, but still relatively slow (for whatever reason). As I said, this is pure speculation.
Also, I’ve been getting more complaints about laggy watchfaces in recent weeks (especially with the Watch 5 Pro). Anyone with the same experience? 
Matze
