Of course, if you're really concerned about it, you can easily put together a test. I suspect other latencies in your app would be larger than those due to the difference between integer lookup and text lookup. But like I said: if you're highly sensitive to performance, you'll want to be measuring against your application, your stack, regardless of advice you might read about on the internet.

If beautiful URIs are a priority, you can use a separate mapping table or service (memcache? Redis?) that maps the URI to a database ID. Of course, this adds latency/overhead as well. This could also give you flexibility in supporting multiple slugs for a single story.