Hello, I bought a Cannondale Synapse, mi first serious road bike. The bika came with a FSA 50/34 crank, since the rest of the group is 105 and I wanted bigger rings, the LBS installed a Shimano 105 crank with bb30 adapters

I did not have a clue about the need of adapters if I wanted to install a Shimano crankset.

is the use of adapters going to affect the performance of the bike? is better to use a BB30 crankset?

A different LBS is telling me a BB30 bracket is going to perform better with a BB30 crank, since its going to keep the original stiffness