Normally, in a bash script, exec replaces the bash process, and the rest of the script is ignored:

#!/bin/bash
exec command1
command2        # will not execute

However, if we add & after the exec line, it will continue to run:

#!/bin/bash
exec command1 &
command2        # will execute

It seems totally equivalent to a non-exec script which simply puts command1 to background:

#!/bin/bash
command1 &
command2        # will execute

Is it really no difference between using and not using exec on a background command?

Will it be faster or slower if using exec?