\newcount\cnt\newcount\cnta
\cnt=0
\long\def\loop#1\repeat{\def\body{#1}\iterate}
-\loop\ifnum\cnt<10 % <= this value
+\loop\ifnum\cnt<20 % <= this value
\cnta=0 \message{\the\cnt:}\par
{\loop\ifnum\cnta<500 あ.「い,うえお・か(き)く)(けこ\advance\cnta1\repeat}
\advance\cnt1
\end
% real time:
-% 20: 9.950s 9.214
-% 10: 5.209s 5.119
+% 20: 9.950s 10.584
+% 10: 5.209s 5.523
% env: C2D E7300, Mem 4GB, LuaTeX 0.71.0pre, Gentoo amd64 unstable
\ No newline at end of file