I recently flew East out of SeaTac in the afternoon, which is supposed a slower time. Well, it was slow going through the security lines, which include the new "show your junk" scanners. Add in time for the usual check-in headaches plus a potential shuttle train ride out to the S terminal, and your pre-flight trip to the rest room may evaporate!
I arrived about 95 minutes before take-off and I arrived at my gate just as my boarding zone was being called. Sure, I had another 15 minutes before the door closed but as is I just barely got my small (legit) carry-on bag into the tight overhead.
I imagine that one needs a full 2 hours during the morning rush. Whew!