First, make sure your car rental agreement allows you drive outside California.

I will also avoid the traffic peak hours (7 AM - 9 AM, 4PM - 7 PM). It can easily take you 2 hours just to cross from LAX to San Bernadino (I-10/I-15 intersection) with traffic. From San Bernadino to Las Vegas strip, if you just follow the traffic flow (which is about 80 miles per hour), you will get to Las Vegas less than three hours. (around 200 miles)
Listen to the traffic radio if the traffic gets heavy, it will save you a lot of time.