Really good question, I have not actually tested this out yet, however in the past, with the way Python and even Java work, they were able to really deliver superior performance because they able to split the application's processing and memory needs across their cluster. This came with some limitations of not being able to use every library out there, however it did buy some real performance advantages.
With PHP and WordPress I am suspecting that they are doing something a little differently than what was done with Python and Java and perhaps making it more akin to a cloud host like Heroku that has various underlying workers for each php process rather than being able to actually multithread the app itself.
It definitely merits some benchmarks, but even if we don't see the kinds of performance improvements that Python and Java saw, Google's infrastructure alone is no slouch and something not to be overlooked. This certainly could be a game changer if Google has done it "right."