they are very heavy, the game uses a thrust/mass formula to determine your maneuverability and top speed. So by increasing mass, you reduce overall speed and maneuverability.
1. Shield recharge has a constant absolute value which has been experimentally shown to be 1MJ/s for any ship and any shield.
2. The optimal and max mass values shown on shield generators only ever refer to the naked hull mass. Adding or removing mass to/from a ship has no effect on shield strength.