> but if there was a notable speed boost for having quick access to the first few bytes
Few programs would get anything out of early access to the first few bytes of a longer file. Most programs dealing in text or small binary records use fread() or similar to make full block reads delivering convenient quantities without bothering the kernel repeatedly. fread() will read the whole first block and wait for it.
The kernel deals in whole pages making a valuable correspondence between mapped memory and file buffering. Space in a page past the end of a file can be all bytes zero. Space in a page which is unknown because it hasn't been read yet is complicated.