Looking at the traffic screen on google maps last few days this seems like easiest way (assuming you are starting close to 15)

I15N to 91W to 71N to 57N to 210W to 5N!

(Temecula-Corona-Chino Valley-Pomona-San Dimas-Pasadena to 5)

About ~ 10 miles longer but seems a lot less congested!