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"-ab" and "-b" is set with "k" not "kb", the first command is just wrong
My best guess is that when you use "kb" the setting is ignored and it keeps the original audio and video bitrate, then if you use "k" it actually changes the bitrate to your setting and this is why it also takes longer, so it's pretty much if you want it to work you have to use "k"
You can use preset to set the speed of encoding, with "-preset fast" it will go faster but the final quality will be worse, "-preset slow" will take more time but will produce better quality even though both files will be the same size. You can try different presets and find one that gives you the best speed to quality ratio that you can tolerate.
Presets you can use:
ultrafast, superfast, veryfast, faster, fast, medium, slow, slower, veryslow
However on another note, the whole thing of setting the video rate for mpeg4 is slightly wrong as the format itself is designed for variable bitrate (still moments in video will have low rate while fast motion will have higher rate), if you choose constant the quality of the video will vary based on how fast the motion is and in some very fast phased moments it can be really bad, then on slow phased frames it just waste kbits for nothing; with variable rate it will keep pretty much the same quality regardless of the motion. (imagine in a hardcore scene it shows a close up of the dick hammering the pussy real hard and the quality at that moment gets really bad and grainy...)
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