Hi, I have a script that instantiates chain links.
the script does nothing with the physics accept for change the connectedJoint. and when large numbers of chain links it seems to slow down(the frameRate is normal though) and i don't know why it would slow down like that.
here is a of the settings for the joint holding the chain links together. 
Hi,
It's likely that you will need to play around with the solver iteration count of the physics settings AND the time step of the time settings.
I found that on windows the physics could really get very slow, but on mac, it doesn't happen, and therefore I have some checks to adjust the solver iteration count based on he platform.
Also, the collider you use could be partially responsible, for this I haven't made any benchmark, but I am sure plays an very important role for large numbers of calculations.
Hope it helps,
Jean