A computer power supply is not really designed to charge accu's.
BTW is this an old one you have laying around , or is it feeding your computer as well ? In the second case you may be overloading your power supply, hence reduced output current.
A charged 12 Volts accu sits at 12.6 Volts with no load connected to it. To charge it properly your charging voltage should be above that. For slow charging opt for about 13.5 to 13.7 Volts, for fast charging max voltage up to 14.4 Volts.
If you supply output is 13.5 volts it should charge your 12 Volts accu.
Put a 12 Volts 21 watt car bulb in series, it will initially reduce your charging current. As the accu voltage builds up the volts drop across the bulb will be minimal, certainly less than a rectifer diode.