Delay a request via webrick

code :

require ‘webrick/httpproxy’
class MyServer < WEBrick::HTTPProxyServer
def do_GET(req,res)
if res.path == a string
sleep 30
res.body = a string
else
super(req,res)
end
end
server = MyServer.new :Port=>8000
server.start

I want to delay specific request,but it delay all request.why?how to
resolve it.