If you have time to pop onto SK every few hours during the day so you can feed your sprite whenever it has full hunger again, it should go faster. My maskeraith is still level 65 or so(4* currently), but some weeks I don't have time to get on a lot and feed it, and I was using 2* mats for a while after I went through my limited stock of 4* and 5* mats. I'm using 4* mats again after crafting all of the orbs that I would need so it's going faster. Buying or trading for sprite food would help, as would buying/doing runs for high-star mats.

I've gotten my maskeraith to level 97 by doing nothing more than pumping it full of every day materials and required food. And I'm not rich. Ever since I got them I have been feeding my pets like this:

Levels 1-14=Any spare materials under 4*. They raise heat quick enough and I'm never at a threat of loosing to die for materials.

Levels 15-49= I will now make pet food when ever possible as feeding them random materials isn't quite so useful. I will now feed it 4* materials if need be, but other then that their diets stay the same.

Levels 50-89= This is where things start getting complicated. I will feed them the needed pet food and throw in 3* or 4* materials to fill up spare appetite. If I can spare some 5* materials I will use those too but not often.

Levels 90+= As stated before, at this current moment my maskeraith is level 97. Ever since I began hitting ultimate skills I have been feeding it shadow stars 90% of the time. The other 10% is any 4* materials I can find an abundance of. If Shaderon could talk, he wouldn't be able to because I shattered his teeth with rocky cores. I keep most 5* materials for making the pet food. I'd prefer 4x the heat for 2 materials than hardly anything for 1 material.

My sprite still level 56(we have similarities..!) and as people suggest, I only fed 4* mats from now on. Crimsonites are just uncommon and I found Dark Matters everywhere so you might want to keep the crystal until your sprite is level 90-ish. (if your sprite is Drakon)