Well, not really. Slow presets encode slower, but with a better compression, or, for the same compression, a better quality. So, if you have enough time and if disc space is important, you may want to use a slow preset. But take in mind that if you select a slow preset (slower than the default "medium"), the encoder will probably use a higher level automatically. If compatibility is important, you may have to select the Force level 4.1 option, and doing so restrict somewhat the power of the slow presets.
IMO, the default value (preset medium) is very well chosen by the x264 team. It is a good balance between speed, compression ration, quality and compatibility. It's only because I prefer smaller files that I encode usually with the preset Slower (and force level 4.1). I have tried to use also the veryslow and placebo presets, but they are really too slow for my PC, and the benefit is not very important. Sometimes, I use also fast presets (even ultrafast) to do quick tests, when disc space doesn't matter.
Note that in CRF or CQ mode, changing the preset affects mainly the file size, while when you encode in ABR or 2-pass, since the bitrate is fixed, it's the quality that is affected.
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