Should I use a quick or gradual 90 in my flexible A/C ductwork?
Sharp corners in the duct work are not preferred. They create eddy currents in the air flow that results in greater pressure drop than a straight run of pipe. In some situations the eddy currents can also create increased air turbulence noise as well.

Instead of using elbows for ducts you should consider using the newer style of non-metallic duct material. This has many advantages including:


It is much easier to work with.
There is far less thermal drop across the duct material than metal
duct.
It is lightweight.
You can easily snake it in any direction during installation.
The insulation material is built right into the duct itself as
opposed to having to be installed separately.
Duct tape sticks to it like a champ.
Tools needed for installation are very simple ones that most DIYers
will already have.