It depends. Stacking cables generally increase bandwidth between stack members; much as a chassis switch fabric increases bandwidth between line cards within the chassis. However, unlike a chassis fabric, stacking is generally physically a daisy chain, so latency between "far" apart switches might be increased. Additionally, low volume traffic between a pair of switches might be delayed by high volume traffic between another pair of switches. Lastly, how well a stack performs depends much on its architecture. For example, compare the original 3750 stack architecture vs. the later 3750E Plus" architecture. The former placed ALL switch traffic on the stack ring, the latter only placed unicast traffic, that needed to go to another switch on the stack ring. The former sending switch took the traffic back off the ring, while the later receiving switch took traffic on the ring.
So, given the foregoing, you cannot easily say whether there always is any increase in switching capacity and/or forwarding rate. However, one advantage of a switch stack, it logically becomes just one device which has many possible advantages vs. separate switch units. For example, rather than two L3 switches running a FHRP, the L3 stack's gateway is across all the stack members using the same VLAN (failover, by default, is usually much faster too).
