First, you need to get the tranny fluid leak fixed -- as long as the fluid is low, the tranny will slip and shift slowly since compressed air and oil foam doesn't act like pure hydraulic oil in the clutches. You will eventually also ruin the tranny from the slip.
After you get the leak fixed, you will need to re-adjust the linkage for proper shift timing and feel. You didn't list the year, so I don't know if you have a modulator or not.
Check the tranny cooler lines for leaks -- the one on my 220D wore through because the rubber in one of the clips wore out, and it only leaked while the engine was producing power -- that is, only while accelerating. At rest, the line sat on the hole and nothing came out! No drips in the drive, nothing, but it must have been pouring out while I drove!
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!