If you assign common words or syllables to additional chords, then speed goes back up, but it becomes so much harder to learn -- so much memorization.

Stenographers (like in court) use a keyboard that relies on simultaneous keys (the chord principle, but still with more keys than fingers) to achieve speeds even greater than normal speech, with their own predefined combinations (tons of practice to learn) -- but unless you're transcribing speech in court or providing live closed captioning, it doesn't seem to be a skill worth investing in.