Forum OpenACS Q&A: Response to Impact of Exec on server performance
Have you looked into using TclCurl? If it provides the functionality you want, it might be worth your trouble to get TclCurl working within AOLserver. This would at least get rid of the exec overhead.
However, if you're using cURL to retrieve resources from elsewhere on the web, that download time may turn out to be much more significant than exec overhead, meaning that TclCurl won't be any sort of silver bullet.