Having lived in both places, LA traffic is universally more oppressive than San Diego’s.
It permeates well beyond the famed 405. Surface streets on the Westside for example are just as clogged.
It can take 20-40 minutes to travel 3-5 miles in some areas on surface streets.
That said, LA is not one monolithic block of concrete as it used to appear to me as a San Diegan when my experiences were driving through to Santa Barbara or spending a weekend here or there doing LA stuff. It is really an amalgamation of unique neighborhoods … some of which have a lot of character and even a small-town feel and other areas that are shady, glitzy, ethnic, etc.
Many parts of LA are indeed hellholes. But for every 5 or 6 hellhole neighborhoods there is a gem. And like any precious gem, they are expensive.