What's the lightest\fastest rom available? (XT01550)

Hi everyone, so, i JUST got a New G3 2 days ago, and i have it unlocked, there is just too many Roms and they are not really sorted to know which is optimized best for what. tho i have the 2Gb Variant, it does noticeably lag in the UI and general use, and that's worrying me because the phone is still brand new, i only have TWO apps installed (whatsapp & instagram) and when i open whatsapp for example, i just see a white screen for like 3 seconds before it opens, instagram also works fine for like 3 min, but if i just scroll down too for a while, it starts lagging horribly then force close, i had 5.1 LP, then upgraded to MM 6.0, it was even slower and worse battery, so i downgraded back to 5.1, so i want to get a custom but a light Rom to help with performance best than anything else.

thanks in advance.