The current version of mkusb-dus, 12.4.7, creates persistent live drives, that can help during iso-testing because you can skip testing the integrity of the drive, which can last for minutes in slow USB drives.
So if you use the default ‘persistent live’ menu entry for Lubuntu with Calamares or the ‘live-only to RAM’ for the other Ubuntu flavours with Ubiquity it should work faster than with a standard live drive.
The next step might be to create an extra menuentry in the standard
grub.cfg for this purpose (unless, of course, you would skip testing in the default boot option). I will come back and suggest some additional menu entries …