Whether we are talking feather pillows or pillows and cushions with sythetic fibre stuffing, I do the following.
Wash them on a 30 or 40 degree cycle that spins them at a medium setting.
Then put them in the tumbler dryer with at least six tennis balls or plastic balls (any kind will do, you don't need the ones specifically marketed for use in dryers.)
The balls make a dreadful racket while the pillows are drying, but ignore that. They stop the stuffing in the pillows becoming lumpy.
Dry your pillows and cushions on a high setting. If when the cycle ends, they are not quite dry, dry them again.
In this heat, if they are only slightly damp, lay them outside full in the sun and turn them every quarter of an hour until they are dry.
I know dryers use a lot of electricity, but there is no way of not getting lumpy pillows unless you dry them in the dryer. It is after all only once a year most of us wash pillows.
If the capacity of your washing machine and dryer are both big enough you can wash and dry duvets, quilts and down -filled winter outdoor clothing in the same fashion.
If you don't have a dryer, or cannot afford to use it, the only way you can avoid lumpy pillows and cushions is by opening them up and taking the stuffing out, letting it dry in the sun, then beating or teasing the lumps out of it.
This is difficult with feather pillows -as the feathers as they dry will fly away! Sythetic stuffings or cotton wadding can be washed and dried in this way, but you have to dry it quickly - if it takes more than a few hours to dry, it will smell musty.