First I developed 1-at-a-time myself, only I performed it at the M-slice. Then I evolved that into 2-at-a-time.
I then saw the movie about 2-at-a-time and 6-at-a-time on the E-slice and learned those. Doing it at the E-slice gives better look ahead for me, so I do that now.
I used 6-at-a-time for about 6 months, but my total times never got below 2.10.
I have been doing 2-at-a-time for about 2 months now and my total times have dropped about 25 seconds on average. I think about 10-15 seconds of this has to do with better look ahead and no "strange" situations. Current times: 1.30 (best) 1.50 (average), with keyhole + 4 look last layer (sometimes 6 look if I have really bad parities)
Advantages of 2-at-a-time: Simpler, more of the same moves, better look-ahead, no strange cases
Advantages of 4-at-a-time: Less setup moves, less slice/matchup moves, less locations to look at after the first matches