Random keys which are combinations of alpha numeric characters and lower and upper cases, are considered to be stronger and secure. Totally random keys however have an inherent problem of being difficult to remember. Also, generally speaking, the longer the key, the more secure it is. However, the maximum allowed preshare key length on a cisco router is 128 bytes. The IPSec peers use preshared keys as one of the methods to authenticate each other during IKE phase 1 negotiations. This does not happen too often. Thus, it is my guess that long preshare key length will not affect the performance too much.
