Deep washboards are tough to keep any real speed through. I definitely have run into washboards that have forced me to reduce my speed more than rocks and ruts.
If feasible, loosen the heck out of your grip in the bars, so the front end can bounce in control. Think of yourself as suspension.
Other times, you just have to grab two fistfuls of brakes and scrub as much speed as you can to get away from the jackhammer.
Try to find a line that reduces quantity you hit. Usually on the edges but the dirt is softer there, so be ready for that.
Zig zag can help you hit them less square. Guarantee that in the middle of nowhere, as soon as you start zigging, a big white truck will be right behind you.
Practice. I think your brain will be fine.