Ok, I am going to give an example with some numbers. I use my son because I am familiar with what his numbers are. I am getting my numbers and descriptions from his Perfect Game profile page.
Here are straight numbers from the tournaments he has played in:
FB:85, CB:71, SL:70, CH:72
FB:84, CB:71
FB:83, SL:70
FB:84, CB:71, SL:74
FB:84, CB:72, CH: 75
FB:87, CB:81, SL:80
FB:87, CB,71, SL:71, CH:76
FB:86, CB:72, SL:71
Finally, here are numbers and a description from his 2 PG showcases he has done:
FB:86, CB:71, CH:78
"feel for 10 to 4 curveball"
FB:86, SL:70, CH:76
"nasty 10-4 slider with very good late life"
OK, at least according to PG, who is supposed to be the premier scouting organization in the country, velocity doesn't matter when it comes to the difference between a slider and a CB. I know that my son can throw this pitch with a more loopy action with tons of movement that appears slower or with a more late breaking with less movement (but still a pretty good bit) that appears to be thrown harder. The numbers don't show that. Looks like both travel at the same speed and sometimes are called a slider and sometimes are called a CB.
Apparently even the experts are unsure and apparently, velocity doesn't really matter in determining what the pitch is called. Maybe it's just movement. You make some good points GBM, but apparently not everyone is on the same page.
Very interesting debate.