Now is the best time to plant it in the ground, but it should have fully mature fronds before you even consider planting it in the ground.
As for it growing slowly, they definitely need sun and when hot they definitely like some additional moisture. If your doing all of that and still not getting results then maybe its time to repot. Only repot if it comes out of it's pot easily with the roots wrapped around the dirt. If the roots arent fully wrapped around the dirt then it can still grow in that pot longer and taking it out of the pot can cause root damage and set it back a lot further.

Good luck! Hope it grows fast for you again. I have had terrible luck with robustas in pots, they are happier in the ground but defintiely have to be a good size before being planted in the ground in cooler zones.

-Alex

A seedling that young shouldn't need repotting yet. Alex is correct. You basically want the palm to be nearly root bound, so it holds the soil in place, before transplanting to a bigger pot. Give it as much sun as possible this summer. But if it's not out in direct sun already, don't let it get sunburned! Start slowly by putting it in filtered light under trees or shrubs first and gradually bring it into full sun over a period of a week or so. All palms go through periods of growth and rest. Be patient.

hey cool thanks, yeah i think im going to wait it out. and repot it soon. Im just afraid that the stray muts around here might get to it and pee on it like they do to my flowers. hahaha Anyway how can you tell if the roots are completly wraped around the soil. and We get very serious monsoon season comming up here soon with alot of rain would that be good for the palm/ I dont want to drown the poor thing it took me all this time to get where it is now so ill just have to wait i guess. Thanks maybe next summer it will be somewhat fully grown to plant into the earth. cant wait. lol

If you see roots coming out of the bottom drain holes in the pot, perhaps it's time to repot.

kaotickelly-Your palm will handle our monsoon. Washingtonia naturally grow in wet areas(at least underground). That being said. Robusta are really only doable in Las Cruces/Alamogordo and may persist 3-5 years in ABQ area without protection. You state your zone as 5b which really is pushing it, meaning you will need to protect. In Albuquerque, washingtonia filifera is the best washingtonia. They seem more successful when planted with a couple feet of trunk. I am in Rio Rancho and I would consider it about the limit for filifera w/o protection.