Well, it runs stock Lollipop, and it would have been the reference device for the OS, so would have the best optimisations IMO. Even in reviews I've read of the G3, they've compared the speed to the Nexus 5 and M8 (benchmark for skinned Andorid).
The G3 wasn't bad, though.
I am very impressed with the G3, so much so I am considering an LG for my upgrade later this year.
Since Samsung has ditched snapdragon chipsets for their newer phones custom ROM development will be slow. The ROM development and android upgrades for my S4 was so painfully slow that I decided to get myself the Nexus 5 for my birthday. The Gflex 2 and the G4 should both have snapdragon chipsets so there will be more custom ROMs available for them.