It seems that the older Hitachi HD44700A00 chips were slower than newer versions.
I have found at least one timing parameter in the DMC manual that is worse than those shown on the more or less 'standard' Hitachi HD44780U datasheet on which the LiquidCrystal library delays are based. The minimum cycle time (between Enable pulses) specified in the Optrex data book is 1000nSec whereas the later Hitachi data sheet shows 500nSec.
The date code on mine is 9885RI, so it was made in August 1998.
Mine, which are probably 10 years older, have a different form of date code. Several are '0H3' and others are '6M3' '0C1', and '4K18'.
Don