FYI, it seems that the J1 is basically incapable of properly printing via USB (eg Octoprint, Luban) control. The net result of this is the printer slowing down during a print or even pausing (See this video).
You can also see evidence of this in the terminal or logs - a message like statistics_slowdown_cnt 451749 will appear (Meaning that it has slowed down 451,749 times due to the buffer getting too low). This is code which has been added by Snapmaker to debug this issue
Snapmaker have admitted as much to me in an email:
I was speaking with a Marlin dev about this, and he seems to think though that the main issue is that SM have used too low values in the Marlin config - see here
I am probably going to try and build and flash the changes suggested by the Marlin dev to see if I can rectify this issue, but I may not have time for a while. If anybody else can repro and feels like flashing custom firmware, please let me know your findings…