Actually coming I-5 south that time of day isnt that bad... getting out of the SEA/Tacoma/Oly mess will be key (leave as early as you can) but as most turds (Kevbone, etc ) live in Vancouver and commute into PDX to work they are all headed North that time of day... hardly anyone lives in PDX and works in Vancouver (i.e. headed south at that time of day). You might catch a little congestion but nothing like the property tax duckers headed north
Honestly unless you get delayed getting out SEA/etc you should be in PDX in 3.5 easy... just listen to the radio when you are 1/2 hour out and see if anything is messed up...
Where in PDX are you headed? This will matter...