Your car has a four speed transmission. First gear is engaged only at a
full stop, not 3 mph. Or at a full stop, one can shift into first by
placing the shifter into L and taking off, briefly upshift to S and then
immediately shift back to L to hold second gear, then back to S for
third and D for fourth. The point of all this is to ensure that your car
is using all of its four gears.
The second aspect is the accelerator linkage - a poorly adjusted linkage
will make the most willing engine a slug. With the engine OFF, have an
assistant sit in the car and operate the accelerator. Your assistant
should floor the accelerator, you watch the linkage and observe if the
linkage achieves its full throttle stop (look behind the injection pump,
use a flashlight) if not, the linkage needs to be adjusted.
You can try this, but keep notes as to what you do so the original
settings can be restored. I found the pressure rod (between the fire
wall and the linkage on the engine) to be the most sensitive - there's a
pressure rod connection with an adjustment bolt near the fire wall -
open the bolt and put the linkage under some pressure, not slack.
tighten the bolt and test drive the car - it won't be a Porsche, but
should be better.
You don't say which year 300SD you own. Earlier cars were pretty simple,
later model years have some vacuum connections to control the turbo's
wastegate etc. that may be dirty and / or leaking so that's something to
look at as well if your car is of the later vintage.
You should also know that a diesel, due to its relatively heavy internal
parts, is more of a draft horse than a race horse. This car has a 0 - 60
acceleration time of about 13 - 14 seconds, adequate for most driving,
but certainly not fast by today's standards. And that's how it is.