The free Remote app and the inexpensive Keynote Remote app are both stable and fast, assuming you’ve got them working on your network. Many folks have had difficulty getting past the authorization screens of both iTunes and Keynote (and any other iPhone-to-app sync programs like Delicious Library), even when meeting the requirement that the host computer and the iPhone are on the same network (NB: I use iPhone, iPad, and iPod interchangeably with respect to the remote-control apps). There are a few rules to follow to ensure they work:

1. If iTunes, Keynote, or any other program running on the Mac asks you to allow incoming connections, click “Allow.”
2. Your Mac must be on the same LAN (wired or wireless or both) as the iPhone.
3. Your router’s firewall must not filter WAN NAT redirection.
   
4. If you use a third-party firewall on your Mac like Little Snitch, ensure that the firewall rules permit the Keynote, iTunes, and other remotely-controlled or synced apps are permitted to communicate with the local network (both the default method (IPv4) and the local network/IPv6 (yup, IPv6)).