I travel there constantly. You will always get slow traffic on both sides around Baldwin Park ave due to the mall traffic.
There is always a slow and dangerous transition where the 210 turns right and the 134 goes straight ahead and then there are some hills. Once you get into the San Fernando Valley, continue the 210 until the 5. Use the truck lanes if you can. Then go through Santa Clarita to the 126 (just past Magic Mountain), then take the 126 to the 101.
Best time through the Pasadena area is between 10AM and 2PM. That's for either direction.
I almost always stop in Filmore to stretch my legs and visit Starbucks.
Yes, the 138 route will avoid all the town traffic. 138 is mostly flat open desert. The 15 is a slow uphill with construction and the 14 is a long downhill.