I have a 77 with an 89 3.2 in it. When I got the car it was slow too. It actually ran surprisingly well but was, well, slow. It turned out to be many things:
1) The fuel filter was clogged
2) The 2.7 CIS fuel accumulator was still installed in the fuel system
3) The CIS fuel pump was providing incorrect fuel pressure to the Motronic system
4) One of the plug wires was severely corroded
5) The CO setting was way off
6) The intake boot clamp was restricting the throttle to about 80% open
7) The "brown wire" was jumpered on the ECU
I added a Steve Wong chip and cat bypass and now the car is as fast as I expected it to be. Fast enough to whack the ass end sideways in second gear on a freeway on ramp.
I would suggest you check for fully opening throttle, throttle position switch sensing full open, correct fuel pressure, correct CO setting. Also do a compression or leakdown test to see if you are developing normal compression in all cylinders.
Steve Wong has a good description of how to check the throttle position switch. I think it's on his website
911CHIPS - Performance Chip Tuning for the Porsche 911