Howcome ?

[Zazu45]

Hi guys.
I recently shot a few patterns thru the "UNDERTAKER" with BB's and T shot. Results were not too bad , but nothing to jump up and down.
One more question bothers me, everybody say that speed is very important with steel shot and still when I look at what companies offer they are many loads in 1300fps or less.
Even the big 10ga. wich many say is much superior to the 12ga. shows some pretty conventional speeds.
I understand that as the shot gets bigger they can hold speed better being heavier but how slow can you go?
So my question is :what should be the minimum speed one should consider depending on shot size?