It has little affect on the speed or sensitivity except for extreme values. This parameter specifies how many. This is the number of diagonals around each of the 'best' diagonals that will be used. The default for proteins in version 1.8 is the PAM series derived by Gonnet and colleagues. The actual matrix that is used depends on how similar the sequences to be aligned at this alignment step are.
Different matrices work differently at each evolutionary distance.
Further help is offered in the weight matrix menu.