the buffer we are talking about is the UART buffer right?
Yes.
What other data is stored in that buffer?
Nothing, nada, zip, zilch.
Would decreasing the size of the buffer to the size of frame decrease the time needed to send the package?
No. Writing 8 letters on a sheet of paper takes the same amount of time, regardless of the size of the paper, doesn't it? Writing 8 bytes into the buffer, and then waiting for the buffer to empty takes the same amount of time if the buffer is 10 characters as it does if the buffer is 64 characters.