It certainly sounds slow...

With 2.3 hawtness there's no reason at level 20+ to be getting a level each every play session imo.

Are you questing? You should be imo, there's more quests than you can ever do I found, and again - new xp rewards are massive.

Are you grinding? Ugh. Of course it can be done, but I'd venture the only way it'd be viable is with huge AoE.

I'd certainly suggest Instancing if you find your setup is able to. Check our wiki for instance/level guides. Especially instance to complete the quests in each, those in particular have fantastic xp/rewards now that 2.3 revamped so much.

Do a search for leveling guides too, they are almost all written from solo perspective but there's no reason you can't use them as a template for 5-man leveling. I use Team iDemise, Joanna's etc. They can be found all over the place *cough*