Unusual slowness might be experienced if the battery is dead or missing, as the embedded controller / SMC will assert the CPUs bi-directional PROCHOT signal and

Unusual slowness might be experienced if the battery is dead or missing, as the embedded controller / SMC will assert the CPUs bi-directional PROCHOT signal and with that, lock it to the lowest available clock speed (700 MHz-ish on a 2nd gen Core i7, sitting in a 2011 MBP). If you coerce it to... not do that, the laptop might reset whenever power usage goes up quickly, because it was designed with the expectation that the battery will jump in and act as a reserve.