Random helps you learn some of the more unusual note patterns that you wouldn't usually. High speed makes it easier to get higher accuracy, and makes sightreading patterns easier because there's less on the screen to confuse you. Slow speeds will help you see the shape of patterns much easier, and makes SV changes much easier to handle, and can help you earn to rely on the music for timing, rather than watching the notes as much.
I honestly think that every time you decide to make a change to how you play (such as deciding to learn to play at slow speeds, or switching to high speeds instead), you have a much better chance of improving. Every time I've made a change to how I play, once I adjusted to the new style I kept improving until I was better than I was before.
When you find a speed that you feel comfortable at, there's a quick way to learn what speed you need to use depending on the bpm of the song. Take whatever your scroll speed is, and multiply it by the bpm. That number is your ideal speed. That's the number xxbidiao mentioned (C1500 to C2500).
For example, my ideal speed used to be scroll speed 17 at 150 bpm. That works out to C2550. At 140 bpm, I would adjust to speed 18. 140x18 = 2520, very close to 2550. A good rule of thumb is that for every 10 bpm faster or slower than the bpm you used to calculate your ideal speed, you adjust your scroll rate up or down 1 to compensate.
This breaks down when you start reaching uch higher or much lower bpms.
