Q: Slew Rate of MCP601
The op-amp output has to go from saturation at 0V to +0.6 instantly to keep the rectified output from swinging below zero. That's because of the diode forward drop. 

The op-amp recovery time from saturation is not specified by the slew rate, and in fact it's seldom specified at all. It can be much, much worse than you would think from the slew rate. 

Eg. LMV2011 with slew rate of 4V/us but \$t_{rec}\$ recovery time from saturation of 50ms typical.