The performance of speech recognition systems is usually evaluated in terms of accuracy and speed.
Vocabulary is hard to recognize if it contains confusable words: e.g. the 26 letters of the English alphabet are difficult to discriminate because they are confusable words (most notoriously, the E-set: "B, C, D, E, G, P, T, V, Z"); an 8% error rate is considered good for this vocabulary.