I live in Temecula and I have family in Indio, so I have traveled this route a few thousand times. I always go through Hemet (79 off the 10, Sanderson by Domenegoni reservoir coming out to Winchester and then going left into Temecula). The traffic through Hemet isn't bad and the mountains you go through aren't bad at all (not curvy or winding). The problem actually comes in when you get to Temecula. The traffic is just terrible. I've lived here for 25 years and I am ready to move b/c of the traffic. I would do all I could to avoid going through town during rush hour. I would also just stay on Winchester straight and get on I-15 South and then exit at 79 S. and go left, then left onto Pechanga Parkway and go straight down to Pechanga. I am currently on a cross country trip in my 40 foot RV and Pechanga still remains in my top 5 RV parks. Please feel free to ask me any other questions about the routes or town or restaurants or things to do, etc.
P.S. I second that Temecula is MUCH nicer than Hemet. Hemet has a serious meth issue in the past couple of years. Crime is much higher.