But the documentation for HTC says that it could slow down things under certain circumstances

yes, if you are in a CGI environment and you can't use filecaching, for whatever reason.
but in that case your script will be slow anyway because the interpreter has to start it all over again every request, so the speed difference won't be that big compared to the overall execution time.
and last but not least, if speed is *that* crucial for you you should switch to mod_perl or FastCGI anyway.