Same as any other CF service under that sort of load: leverage HTTP caching (e.g.
eTags) and server-side caching as well: Query caching and Taffy's
caching hooks, e.g. with ehCache. I would also be sure that your web server layer will support gzip compression if the client requests it.
We haven't seen that sort of load but I've been using Taffy in production for years and years now and it's been rock solid and never the bottleneck in any speed issues. In case you weren't aware, taffy includes response headers that show how much time is spent in each phase... so you can see if your onTaffyRequest method is taking a long time, or the resource, or serialization, etc...