It would be more of a question of is it worth it to take the time, effort, and money to rewrite the code to do this. If you could take 1 routine, write it in C++ and compare, that would be a start. If the C++ is written by a competent C++ programmer, it would probably be somewhat faster in string manipulation (so would C# for that matter). The development effort in C++ (or even Delphi) is likely to take far longer than VFP since these are lower level environments and you need to worry about pointers and memory.
Here's what Microsoft says when comparing the 2 development environments:
"Q. Why would I build a component in Visual FoxPro when I can build the same component in Microsoft Visual Basic or Microsoft Visual C++?
A. Visual FoxPro components are extremely fast, thanks to the data-centric language of Visual FoxPro and its ability to quickly retrieve and manipulate data. Further, Visual FoxPro can build character strings very swiftly. A Visual FoxPro component that works with data and returns HTML will, therefore, be very fast.
Visual FoxPro also has object-oriented programming capabilities, including inheritance. This provides a high degree of reuse across applications. A Visual FoxPro developer can create a set of classes that includes the core functionality of a Web database component, so that code will not need to be rewritten each time a component is needed in a Web application. The developer can simply create a component that inherits the base functionality, and then add application-specific code to it.
Finally, building COM components in Visual FoxPro is an excellent way to reuse existing code. Code that is already written and tested can be built into a component, rather than being recreated in another language."
From: http://msdn.microsoft.com/vfoxpro/productinfo/faq/default.aspx


>Hi to all who are interested in this topic.
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>One of my customers has a colleague who states that the application that I've written should be written in C++, which would give a better performance. I wonder whether he's right and if not, what arguments I could use.
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>The core of the app hardly uses DBMS functions. Instead, it uses low-level functions like FOPEN(), FGETS() and FPUTS(), and string manipulation functions like STRTRAN(). It reads tab delimited files, not with APPEND FROM, but with FGETS(). The reason is that the fields of the import files vary and the file size can be huge. And it also writes files with FPUTS() after having done some sophisticated string manipulation. The speed is still phenominal, thousands of records/lines are processed in a second or so. But the import files can contain 100,000,000 records or even more, so it may still take a while before they're all processed.
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>The big question now is: Will C++ or Delphi do this job significantly faster??