What's the *real* bottleneck?

I love this game, I really do... but why in Drifter's green Duviri do I need so. many. damn. orokin. cells. And don't get me started with Argon Crystals. I just happened to land a resource booster with the login reward and I just spammed Void missions. But is that intended?

Let me know what's your current bottleneck.

Edit: Thank you all for your replies. I'm a returning player, so a lot of this information is super helpful. I've always used Hydron to level up stuff, but I hear a lot of Helene suggestions!