parallelStream for time consuming methods [closed]

I have something like this:

// sequential version
for(var t: tasks){ 
    // run() is a time consuming method (approx. 20-30 seconds)
    t.run();
}

I have approx. 1000 (independent) tasks and the above needs approx. 1000 x 25 seconds. The run()-method is only CPU intensive (there are no I/O Operations involved).

I switched to parallelStream:

// parallel version
tasks.parallelStream().forEach(Task:run);

and measured that with parallelStream each task.run() needs approx. 15-20 seconds. So it's definitely a little faster than using the sequential version.

Can I use parallelStream (with its fork-join) for such long running operations (each task.run() needs approx. 15-20 seconds) or would it be better to use executorServices with custom thread pools (for example, because of system stability, performance, etc.)?

What I want to know if there is any harm when using parallelStream (fork-join) for such long running tasks. I have thought that fork-join should only be used for very short running tasks (max. 1-2 seconds), but I am not sure if so and why.

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