You're right, I think we're using IO/CPU bound in different senses. You're talking about where an average request spends most of its time; I was thinking more in terms of Shopify's scaling bottlenecks. Even though individual requests probably spent a lot of their time in CPU, we didn't really have to worry about CPU as a bottleneck for the larger application. Whereas IO was a different matter, and we had a lot of difficulties ensuring that we had enough capacity for spikes in the IO-heavy stuff. So the overarching Shopify application was "IO bound" in that sense.