I got a similar type of a problem (2.2.1 x86 Apr28 2008 Win32):
In short:
I built second workshop and I supposed production time of all items would be halved. But production time of planes remained unchanged, while production time of most other items was divided by 4 (in place of 2).
Details:
Situation 1
prod. limit:5
workers (ready/all): 10/10
Worker capacity: 10
Situation 2
prod. limit:10
workers (ready/all): 20/20
Worker capacity: 20
Examples of production time (situation 1 / situation 2):
Firebird 336 hours / 336 hours
Saracen 400h/400h
Shiva 101h/26h
Shiva 20 mm rounds 10h/3h
Targeting Computer 572h/143h
Fuel Pod 80h/20h
AA51 launcher 155h/51h
AA51 Cicada 12h/3h
Assault rifle 90h/23h
Ass. rifle magazine 12h/3h