The extension column should be fine.
Could you post a screenshot of your lister? Maybe there's something else turned on which is causing the delay.
MTP only lets you send one file at a time, so if something is triggering lots of files (or one big file) to be downloaded then it can block other things from copying to or from the device.
Make sure the phone is not asleep or screen-locked, too; those can block MTP access and sometimes instead of just failing the API takes a long time to realise instead.
Occasionally the PC side or the phone side seem to get confused and I have to fully exit Opus and restart it, or even reboot the phone. (Can happen with Explorer as well.)
You can set Preferences / Miscellaneous / Advanced: mtp_enable = False to make Opus use Explorer's MTP support instead of its own. With most devices it's not as good but it seems to improve things with some devices.