'So the only effective solution seems to disable the random access request (i.e. remove the FILE_FLAG_RANDOM_ACCESS flag) from the Windows API calls used to create/open the files. Moreover, in this case the file-system cache size limit should not be actual anymore, as Windows won't be expanding the cache out of the reasonable boundaries. The quick tests prove this solution being workable.'

.......

'Taking this into account, as well as the experience of other databases, this solution has been committed into Firebird 2.1.5, Firebird 2.5.2 and Firebird 3.0 branches.'


This made me assume this issue wasn't the cause of our problems.  However by the sounds of it the fix might not be working.
   

We are using Windows 2008 R2 64 bits, with databases around 35 Gb, and we solved the performance problems with FB 2.5.2. We tested it before and after apply the patch in 2.5.2 versión and verify that the problems was gone.

I thin you have to look up in other direction.