Hi Ducandu,
          i  beg to differ. Settling Time is always INVERSELY PROPORTIONAL to the bandwidth of the system...Higher the CMFB BW, faster is the o/p CM settling.
      Further CM Oscillations (i suppose you are referring to the ringing during CM settling) depends directly on Phase Margin of the CMFB Loop.

@ Berti :  say, for example, We consider a generic 2-stage Rz-Cc compensated OTA with a CMFB Control Loop.  Generally CMFB employs one extra Gain Stage and returns the feebback to some point inside the first stage of the OTA(which in most cases is the gate voltage of the Load Transistors in the 1st stage). This extra gain stage of CMFB Loop introduces another Pole into the CMFB Loop, while the tf of the 1st & 2nd stage will retain it's poles and zeros (with slight change in gain coz now we are looking at CM instead of DM) .
BottomLine : You are correct. There is a certain relationship b/w CM & DM BW.
My Question : Without any extra compensation for CMFB Loop(like placing RC compensation post the averaging node), Is CMFB BW always lesser than DM bandwidth ??  

regards,
Mayank.