[SPBS]

I've edited 1GB sql dump files in Vim with no problem. I had to turn off my vimrc for it to work, but 300MB should be no problem as long as you are using barebones vim.

For performant buffer-wide operations (:g, :%s), I had success with using vim like an improved sed by passing in commands directly via the command line. That was noticeably faster than opening vim up and running the commands before exiting e.g.

    # join together lines delimited by LF into one line, then convert all CRLF into LF
    vim -Es \
        -c 'g/[^^M]$/.,/^M$/join' \
        -c ':%s/^M$//g' \
        -c 'wq'