The --limit "io X" option does not behave as expected and seems to not run jobs when the machine is mostly idle.

According to the documentation and comments on --limit:

sub limit($) {
    # Returns:                                                                                                                                                         
    #  0 = Below limit. Start another job.                                                                                                                             
    #  1 = Over limit. Start no jobs.                                                                                                                                  
    #  2 = Kill youngest job        
...
}

However, line 6459 of the parallel script (release 20210222) in function limit() has:

                   exit ($max < '$limit')' $io_file;

which returns a '1' if $max < $limit (True) and hence jobs don't get run.

Should the test be:
                   exit ($max > '$limit')' $io_file;
?