I-15 from So-Cal to Vegas can be a smooth ride or a nightmare, depending on the day of the week, the time you plan on traveling and accidents. We've made the trip many times. The last time was about three weeks ago. The normal six hour drive (from San Diego) took eight hours due to a traffic accident. I-15 N/B, in the middle of the Mojave Desert, was a parking lot for miles - at 9:30 PM in 110 degree heat.
While enroute to Vegas, you'll climb the Cajon pass outside of Riverside, which is a 6% grade that's several miles long, then another grade that's not quite as steep, but also several miles long. Much of the route is two lanes in each direction.
Kev