ParticleSystem.MainModule.startSpeedMultiplier

Description

A multiplier of the initial speed of particles when emitted.

This method is more efficient than accessing the whole curve, if you only want to change the overall speed multiplier.

using UnityEngine;
using System.Collections;

public class ExampleClass : MonoBehaviour
{
    private ParticleSystem ps;
    public float hSliderValue = 1.0F;

    void Start()
    {
        ps = GetComponent<ParticleSystem>();
    }

    void Update()
    {
        var main = ps.main;
        main.startSpeedMultiplier = hSliderValue;
    }

    void OnGUI()
    {
        hSliderValue = GUI.HorizontalSlider(new Rect(25, 45, 100, 30), hSliderValue, 0.0F, 10.0F);
    }
}