For what it's worth, another 2c from another web developer - for me memory isolation and COW for module dependencies would be less useful than a single process and a lack of GIL (hold the flamethrowers for one second please) - allowing me to share on a multicore server resources like those same modules, as well as database connections, as well as any significant static data i may read in at start up, decreasing my operational complexity, etc. In a web server environment shared memory isn't particularly scary, to me anyway. Am I misunderstanding the reason for COW?

Also, the most important characteristic to me is latency. I don't care about throughput that much as long as it's "good enough", hardware is getting really cheap really quickly anyway (in terms of cores and RAM/$). Latency on the other hand is tough. While IO is normally going to be the biggest factor here, if the runtime allows me to consistently shave 5ms off of every request - that's still a great improvement. In this sense i would say that raw speed IS important to me.

Perhaps I haven't encountered the use cases Ian is talking about, it was difficult to figure them out from the blog post (i'm a relative novice in Python).

But I am hoping PyPy can at some point help with the concerns I have above, even lower latency by itself may be compelling enough to switch.