1. show interface will give you what's going on at the time you issue the command. to find out in real time what's going on, use the debug interface X.X
2. frames are at the data link layer frame errors can be that the frame headers the router expects to find aren't what is there, or are missing - and that is just one of many types of errors found at layer 2.
3. yes and no : data at layer 1 is encapsulated into a frame at layer 2, then into a packet at layer 3. these frames and packets are akin to envelopes with data as the contents inside the envelope.

visit Wikipedia for an overview of layer 2 .
http://en.wikipedia.org/wiki/Layer_2

Slow traffic can be caused by many different issues. frame errors can cause a slow down of traffic collisions can also cause substantial slow down of traffic. convergence can cause slow downs. It's up to the network engineer to analyze and determine just what is causing a slow down.