Maybe it's just the picture but the motors so not look to be aligned with the drive shaft? Maybe they are all right but there can be a loss of power there if they are very far off. Try the smaller props first and if possible realign the motors by making the base mounting holes over size in the motor mount base so the whole thing can be moved around (left or right). If necessary, you can shim up the whole mount to bring it into alignment vertically.

The better the alignment the better the motors will perform.

Next I would try 1 3/4" props; you could be loading down the motors thus reducing the RPMs of those 12T Titan 550's.

Bob