The good old speed question/concern ;-)
Well then, when you have an application in VB or VFP which, if I am not mistaken, uses Client/Server side processing, you have the ability to put a lot of load in the client side of things and by doing so, maintain optimum performance. I have seen cases where the old fashion "batch" method is employed where data is batched daily from "client" to 'server' (this is commonly done in portable devices - or so it was back in the 'serial days').
This is not to say you cannot do the same in PHP. If you can afford to batch your data and not maintain a "up to the second" updated files, write your code so that data is kept in the client side in JSON or XML format and sync once daily. Of course, you now are looking at doing some NON-PHP code to pull this off. :-(
Then there is the argument of "executable binaries vs interpreted text like code". VB and/or VFB can be compiled into executable binaries and, if properly done, accessing (or running out of - used to be called memory resident program) client's ROM gives it the added advantage of running at much faster speed even if you were posting to server live.
In the end, it is really a matter of a much larger check list than "speed". There are pros and cons in both - I have seen VFP code choke on simple tasks running in an "intranet" with delays measuring into the seconds so, wait a second here or there really, what difference does it make!?!
I hope you are not using "speed" as the primary criteria to select a development tool. If it is, you may as well look at C, C++ or one of those lower level programming languages. Also, configure your own topology where you can have whatever resources you figure you need for your own application without having to share it with the rest of the world (do not use cloud hosted services - at least none under $1000/month).
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SouthBeach
The good thing about not knowing is the opportunity to learn - Yours truly, 2008.