It is one of the fastest scripting languages, and it will become even faster when JIT is implemented (like an order of magnitude faster). That won't make a difference in web apps because computing is never the bottleneck (it's usually I/O bound : loading 1000s of classes from your favorite framework and querying an API or SQL server that's not even on the same machine comes at a cost), but it could make PHP usable for domains in which nobody would consider using PHP/Python/Ruby today, like heavy scientific computations, image processing, 3d rendering or IA.
Preloading (coming in 7.4) on the other hand could help quite a bit on the I/O side.
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