Starter turns slow - Bosch Bmw Starter 321i 92 93 94 95 2.5l/ 525i 91 92 93 94 95 2.5l/ M3 95 3.0l

You may need a starter bendix. The slow cranking may be unrelated and due to a weak solenoid.

Sometimes battery cables can short the circuit; try getting them away from any metal or each other.

replace starter solinide and check earths etc are clean !

Yep, not enough amps to engage the starter clutch, and when ti does engage, not enough amps to turn the engine. You need a 12 amp battery.

Timing Chain worn out ?

check your battery cables for frayed wires bad connectors solder points at the terminals

Worn carbon brushes and dirty commutator on starter motor.

How was the starting BEFORE all this work was done. Why was the timing belt replaced.

Use a jumper cable from the battery to the post on the starter where the wire from the solonoid connects, if it spools up normal its the cable to the starter motor, if it continues to turn slow remove the starter and clean up the places where the starter and engine case contact each other (to ensure a good ground). If none of these solutions correct the issue the starter could be dragging and need to be replaced.

Sounds like a timing issue. If you have points set them to .018 and then static time the engine.
Hope you have that Shovelhead going by now.