Forcing a CD to be burnt slower will end up with one of 2 things.
Either the disc will not be burnt at the selected speed and will instead write at the minimum speed the drive supports but the software will still tell you its being written slower (check the total burn time)
The drive will switch into using a default strategy for unknown media giving you really poor results.
As for “really slow burning being better”, no. I have found this to only really be true of poor quality media where the disc is off center or wobbles in which case it allows the laser to follow it better. The drawback of using poor quality media is that your data may be fine today or tomorrow but after that you may need to reburn it.
Media quality has more impact on the quality of the burn than forcing very low speeds does, using a default strategy on a good quality disc will result in poor writing performance, increasing the speed so the drive uses a known writing strategy will yeild better results as the drive is not having to second guess how to do things.