So, here's where I break your bubble
The whole process has quite likely taken you at least the same amount of time, or maybe more, as with only the driver
if you converted and then sent books to the device right away. If you converted a bunch and then you're sending a few at a time (like I do) then you may have saved a bit of time. The code for converting on send and converting through calibre is identical; not just copied, but literally the exact same module. It's significantly faster to send a pre-converted KePub, yes, because I coded the driver to not convert KePub books
but there's no time savings at all when you convert and then send.
With all that said... there was some general improvements in the speed of converting books. So it probably was faster to convert than before, but it's not because of using the conversion plugin first