Does Lua not have the startup time issues of Python/Ruby? If it does, then to counteract the lack of threads you'd probably use a coroutine-based system like gevent and Python's greenlets.
matthew@rusticanum:~$ time python2.6 -c 'print("hello world")'
hello world
real 0m0.027s
user 0m0.024s
sys 0m0.004s
matthew@rusticanum:~$ time lua -e 'print("hello world")'
hello world
real 0m0.005s
user 0m0.000s
sys 0m0.000s
That's a good comparison to make, but once you have a Python program or Ruby program importing 20 packages, each with several modules, then it gets really slow. Like 100-500ms, which is more work than the "real" work of a handling many HTTP requests. Bad packages can do arbitrary computation at import time.
To be fair, I haven't run any Lua programs with 20 packages... AFAIK Lua didn't have a module system until Lua 5, and it's not as capable as Python or Ruby's.
