Why PSK31 encodes characters with Varicode, a variable-length code like Morse where the most common characters get the shortest bit patterns. Lowercase letters, which dominate normal English text, get short sequences, while uppercase letters are assigned longer ones. Since the symbol rate is fixed at 31.25 baud, typing in all caps means more bits per character and therefore slower effective throughput.
Watch out Standard PSK31 has no forward error correction at all, which is why a noisy signal produces garbled characters rather than corrected ones; and PSK31's narrow bandwidth means it needs less power than RTTY, not more, for comparable results.
Varicode = common letters short. SHOUTING IN CAPS costs extra bits, so it slows you down.