MBps is considered to be some Million Bytes per second. That's one extreme connection as I used to get about 10 or so Mbps or Million bits per second when I had Comcast.
There is more going on than the 15 Mbps here. Users in your neighborhood share a local network which are now well known to get congested as more folk watch youtube, hulu and netflix. The throttle isn't really Comcast most of the time but a bottleneck of the areas that use a lot of network bits.
I've been wondering why my 15MBps cox cable can't handle youtube very well, even in 480p. I thought it might be a problem with the youtube and not Cox, but I just watched a youtube video on my Mytouch 4g and it never even came close to having to stop and buffer. So, I was curious and put the same video up on my laptop and guess what, it had to stop and buffer 4 time in a 2 min video. So just to make sure I tested it again on my phone, which was fine, and then turned on wifi on my phone and played it again but it had to buffer many times because it wasn't on my wireless carrier network anymore. I know I get around 3-5MBps speeds on my phone around here, so the 15MBps cable internet should never lag my cell phone but it does. Has anyone else seen this?