Simple script to set the playback speed of a particle system on application start Allows you to also lock that speed so it can't be changed by another script.

ParticleSystemSpeed.cs

using UnityEngine;

/// <summary>
/// Simple script to set the playback speed of a particle system on application start
/// Allows you to also lock that speed so it can't be changed by another script.
/// </summary>

[RequireComponent(typeof(ParticleSystem))]
public class ParticleSystemSpeed : MonoBehaviour
{
    public float speed = 1f;
    public bool lockSpeed = false;

    ParticleSystem psys;
    void Start()
    {
        psys = gameObject.GetComponent<ParticleSystem>();
        psys.playbackSpeed = speed;
    }


    void LateUpdate()
    {
        if (!lockSpeed)
            return;

        if (!Mathf.Approximately(psys.playbackSpeed, speed))
        {
            psys.playbackSpeed = speed;
        }
    }
}