Yo!

The -r3mix setting is nowadays a bit 'outdated' (don't shoot me... it's what I've heard..)
Currently the -alt-preset settings are the most tuned one and also activily being observed.

So use lame with "-alt-preset standard"
There are more -alt-preset values, but mentioned one is comparable with -r3mix but slightly better. It produces VBR's with average 170kb, and LAME is OK what about VBR's. See for the other alt-preset values the readme provided by lame.

About the time you need: encoding to mp3 is a time consuming story. de-coding instead is fast.

Good luck,

EiSl