Initially, I had the sam experience and I cancelled my subscription. Decided to give it another chance and, this time, applied the same tweaks that I used in FSX and wow! - what a difference. I have P3D on a q9650 system clocked at 3.88 and FSX on a 980X system clocked at 4.43 and P3D maintains the same smoothness and frames as FSX. I lock mine at 30 so as long as it stays there AND is smooth, I'm a happy camper. I have P3D set up with most of the same add-ons that function in FSX.
So I'd venture a guess that you need to retry P3D and adjust the settings the way you did in FSX.
I7 3770K @ 4.73ghz Asus Sabertooth Z77 16G G.Skill 1600 7-8-7-24 2T 2 x EVGAGTX780 SLI Corsair 1000W PSU 1TB Samsung SSD for P3D - 2 - 256G OCZ Vector SSD - HAF 942 - Corsiar H100i Liquid Cooler W7 64 Home Premium.