Hi there!
I understand that you're having trouble with the Fast Fourier Transform (FFT) in Excel 2007 being slower than in Excel 2003. It sounds like you're concerned that the difference in file format (XLA vs XLAM) might be causing the slowdown.

First, let me explain a bit about the file formats. XLA files are Excel add-ins that are saved in binary format, while XLAM files are add-ins that are saved in XML format. The main difference is that XLAM files can contain VBA code, while XLA files cannot. However, both file formats should be loaded into memory when Excel starts up, so there shouldn't be any significant difference in performance between the two.

That being said, there are a few things you can try to speed up your FFT calculations:

1. Use the latest version of Excel: If you're not already using the latest version of Excel, upgrading to a newer version may help improve performance.
2. Optimize your code: Make sure your VBA code is as efficient as possible. This means minimizing the number of calculations and avoiding unnecessary loops and iterations.
3. Use a different FFT algorithm: Excel's built-in FFT algorithm may not be the fastest one available. There are many third-party FFT algorithms that you can use instead. One popular option is the FFTW library, which has a VBA wrapper that you can use in Excel.
4. Use parallel processing: If you have a multi-core processor, you can use parallel processing to speed up your FFT calculations. This involves splitting the calculations into multiple threads that can run simultaneously on different cores.

I hope these suggestions help you speed up your FFT calculations in Excel 2007.