How fast you heal (or cast any nano for that matter) is dependent on two main things.

1) The skill Nanoinit. in the speed section of your IP investment panel.

2) The setting on your Agg/Def bar.

To cast faster, be on full Agg and invest into NanoInit. ;) Very important to casting is to acquire a Nano Formula Recompiler. It goes in NCU Slot 6 and is vital for any heavy casting class.