http://www.dwelch.com/gba/dhry.htm
※低い数値の方が良い GCC 3.0.4 (homebrew) (ARM) 5724 WAITCNT 3/1 no prefetch 4941 WAITCNT 3/1 prefetch GCC 3.0.4 (homebrew) (THUMB) 4416 WAITCNT 3/1 no prefetch 3668 WAITCNT 3/1 prefetch
GBATEKはこんな感じに書いてあります。
At startup, the default setting is 0000h. Currently manufactured cartridges are using the following settings: WS0/ROM=3,1 clks; SRAM=8 clks; WS2/EEPROM: 8,8 clks; prefetch enabled; that is, WAITCNT=4317h, for more info see "GBA Cartridges" chapter.
「prefetch enabled」をしているかどうかはかなり大きいようです。