Significant slow down when executed in quick succession about isolate HOT 6 CLOSED

sumt1=0
sumt2=0
for ((i=1; i<=$ITERATIONS; i++)); do
    isolate --init >/dev/null
    /usr/bin/time -f %e -o /tmp/time1 isolate --run -- /usr/bin/echo 1 >/dev/null 2>/dev/null
    isolate --cleanup >/dev/null
    isolate --init >/dev/null
    /usr/bin/time -f %e -o /tmp/time2 isolate --run -- /usr/bin/echo 2 >/dev/null 2>/dev/null
    isolate --cleanup >/dev/null
    sumt1=$(echo $sumt1 + $(cat /tmp/time1) | bc)
    sumt2=$(echo $sumt2 + $(cat /tmp/time2) | bc)
    sleep $SLEEPTIME
done
echo sumt1 = $sumt1, sumt2 = $sumt2

$ sudo env ITERATIONS=1000 SLEEPTIME=0.1 bash isolate.sh
sumt1 = 43.54, sumt2 = 99.61

johnchen902@johnchen902 ~> sudo env ITERATIONS=100 SLEEPTIME=1 bash isolate.sh 
sumt1 = 4.34, sumt2 = 84.86

#define _GNU_SOURCE
#include <fcntl.h>
#include <sched.h>
#include <signal.h>
#include <sys/wait.h>
#include <unistd.h>
#define CHILD_STACK_SIZE (1 << 23)
static int fd[2];
static int child_main(void *param) { (void) param;
    // close(fd[1]);
    // execl("/usr/bin/true", "/usr/bin/true", NULL);
    return 0;
}
int main() {
    static int child_stack[CHILD_STACK_SIZE];
    int flags = SIGCHLD | CLONE_NEWCGROUP | CLONE_NEWIPC | CLONE_NEWNET
            | CLONE_NEWNS | CLONE_NEWPID | CLONE_NEWUTS ;
    pipe2(fd, O_CLOEXEC);
    clone(child_main, child_stack + CHILD_STACK_SIZE, flags, NULL);
    close(fd[1]);
    // char buf[1]; read(fd[0], buf, 1);
    // wait(NULL);
}