380kbps will be the absolutely maximum you can possibly expect. But
whether or not you actually get that speed depends on things like:

how far you are from the nearest exchange
how much traffic there is on the internet, especially in your local
area
how busy all the various servers, switches and routers are

and so on. In the same way that a chain is only as strong as its
weakest link, so your internet connection is only as fast as its
slowest router.

You may get slightly faster speeds during off-peak periods: late
afternoon and evening are bad times to try, as your neighbours come
home from work and start watching YouTube videos.

Some providers really do limit bandwidth speeds, usually at peak
periods. The thinking behind this is that it's better to give everyone
a slightly slower service, than have some people unable to log on at
all because of the heavy traffic.

Finally, YouTube is a rapidly growing site -- back in November, they
estimated that 13 hours of video material was being uploaded every 60
seconds, now it's 20 hours in 60 seconds -- and their upload servers
are constantly working at full capacity, no matter how fast they
upgrade. That can also slow things down for you.