I think I see the same effect on fedora! But that may be due to terminal's sequential stdout!

Here is a script which can help you debug this further, it prints start and stop time of shell system command and which cpu the shell was at the beginning and at the end (or at least that's what I believe as I am not into the Linux scheduler at all). I think it is better to rely on these numbers rather than on the frequency you see "spawned 10" on the terminal, something very unreliable! You can process these timings and see for yourself if there is delay from creating to sleeping. I don't see it, e.g. the 1st shell-command is run at 1626196220.813910751 which is, time-wise, near spawned 3 at 1626196220.81309 I also observe that spawned 7 at 1626196220.82161 and start(i=1, tid=7/449711) on cpu 0: 1626196220.828573978 are very near as well.

use threads;
use strict;

use Time::HiRes qw/clock_gettime CLOCK_REALTIME/;

# use this to control which CPU you assign this script
# (and in my system also the shell system command)
#use Sys::CpuAffinity; 
#my $success = Sys::CpuAffinity::setAffinity($$, [1]);

print "starting at ".clock_gettime(CLOCK_REALTIME)."\n";

my @pool;
for(1..50){
    push @pool, threads->create(\&mysub);
}

$_->join for (@pool);
 
sub mysub {
    print "spawned " . threads->tid . " at ".clock_gettime(CLOCK_REALT
+IME)."\n";
    #for(1..100){ select(undef, undef, undef, 0.001); }
    #my $x = 0.0001 + rand(0.0015);
    my $x = 0.001;
    my $id;
    for my $i (1..100){
    $id = "i=$i, tid=".threads->tid;
    print `
        echo -n "start($id/\$\$) on cpu "\`cat /proc/\$\$/stat | cut -
+d' ' -f39\`": "; date +%s.%N
        sleep $x
        echo -n " stop($id/\$\$) on cpu "\`cat /proc/\$\$/stat | cut -
+d' ' -f39\`": "; date +%s.%N
    `;
    }
    print "ended " . threads->tid . " at ".clock_gettime(CLOCK_REALTIM
+E)."\n";
}
 
__END__

starting at 1626196220.80981
spawned 1 at 1626196220.81077
spawned 2 at 1626196220.81192
spawned 3 at 1626196220.81309
spawned 4 at 1626196220.81466
spawned 5 at 1626196220.81614
spawned 6 at 1626196220.818
start(i=1, tid=1/449647) on cpu 3: 1626196220.813910751
 stop(i=1, tid=1/449647) on cpu 3: 1626196220.818011634
start(i=1, tid=2/449649) on cpu 1: 1626196220.815133617
 stop(i=1, tid=2/449649) on cpu 3: 1626196220.819228058
start(i=1, tid=3/449654) on cpu 4: 1626196220.816404247
 stop(i=1, tid=3/449654) on cpu 2: 1626196220.820722313
spawned 7 at 1626196220.82161
spawned 8 at 1626196220.8235
start(i=1, tid=4/449665) on cpu 6: 1626196220.818088441
 stop(i=1, tid=4/449665) on cpu 4: 1626196220.825072117
spawned 9 at 1626196220.82603
spawned 10 at 1626196220.82844
start(i=1, tid=5/449675) on cpu 4: 1626196220.820177902
start(i=1, tid=6/449684) on cpu 4: 1626196220.824213073
...
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bw, bliako