\documentclass{article}
\newcount\n
\newcommand{\astsfast}[1]{%
\n=#1
\begingroup
\aftergroup\edef\aftergroup\asts\aftergroup{%
\loop \ifnum\n>0 \aftergroup*\advance\n-1
\repeat
\aftergroup}\endgroup}
\begin{document}
 \astsfast{10000} 
\end{document}