Heavy traffic on Fridays does start around 12 noon at about the 5 & 134 interchange in Glendale/Griffith Park. It stays heavy all the way down to about the Orange County border where things start to pick up. Here is a tip for you travelling south: When you get to the 10 interchange on the 5 south (right where County USC is (medical campus)), take the 10 East to San Bernardino. Make sure you take the 10 East. Make sure to stay on the right lanes. When you get to the 710 intersection, take the 710 (Long Beach Freeway) South to Long Beach. Once on the 710, merge all the way over to the left lane (the fast lane). Take that all the way to the 5 South interchange (the ramp to the 5 South is on the left). This sets you up on the 5 South in the fast lanes which will eventually turn into a carpool lane once you get to Orange County. I know this may be confusing, but it does save you some time since the 5 South (Golden State) interchange to the 5 South (Santa Ana) gets quite backed up. The heaviest traffic on the 5 South is usually around that interchange. Doing this little short cut should avoid the traffic for you.
BTW, I am a Southern California native and know my shortcuts around the southland.