Watching video is a very data intensive activity regardless of resolution, that being a matter of degree.
With a capped service you are contracting for a set amount of data. Once that amount of data is exhausted your speeds will be throttled to about 150 Kbps. There are two exceptions to this.
One is to purchase additional data known as "Tokens". These are available in differing sizes and prices.
Second is that most plans offer a "SmartBrowse" feature that allows normal or near normal speeds for low intensity activity ... the viewing of simple webpages and checking of email.
If a high intensity activity occurs SmartBrowse will kick out and the FAP speed throttle will activate.