I have driven I15 through the SLC area numerous times. I certainly agree it is a nightmare. Regardless of the traffic the drivers seem to be demons from hell determined to drive over 70 mph at all times. Not only that but the nightmare continues for a great many miles until you have driven well south of Provo.
Fortunately there are alternatives of a sort. You will need to drive south through the Odgen area, then down I215 and cut West on I80. That way you will only have 30 miles or so of insanity as you pass through Odgen.
I80 West is decent and you only need to go a few miles to route 36 south to Eureka. At Eureka you can cut back to the East and rejoin I15 just south of Provo. Another option is to take Route 6 west to Delta and then take route 257 south to Minersville. You can then cut east and rejoin I15 or continue south on route 130 to Cedar City. Regardless of which of the options you take you will not lose too much time.
At one time I would have highly recommended the ride to Delta and then Cedar City. Times and Utah have changed and I find the area very depressing. The beauty of the open areas has been destroyed. On weekends especially, you will see large numbers of Utahns on dune buggies and motor bikes tearing up the landscape. Further south of that you will see huge tracks of land dedicated to wind power and a solar farming.