Stumbled across this, from Forth Dimensions Vol2 (also here on Google docs) - page 7.

Bill Powell suggests a scheme of using a single byte for the most frequently used Forth words. He shifts left, then branches on the carry and then optionally on the sign bit. This gives a two-byte coding for a COLON instruction, and a one-byte coding for 64 literals and 64 fast single-byte p-codes.

He summarises thus:
(By time overhead I think he must mean a time multiplier)

Hope that's a fair rendering of his idea!

Cheers
Ed