As I think you are hinting in your post, the slow response time is probably at least partially due to delays in the system while the OS writes the contents of memory to the pagefile to make room for other processes in physical memory.
The obvious solution (and possibly not practical) is to use less memory in your application. I'll assume that is not an option or at least not a simple option. The alternative is to try to proactively flush data to disk to continually keep available physical memory for other applications to run. You can find the total memory on the machine with GlobalMemoryStatusEx. And GetProcessMemoryInfo will return current information about your own application's memory usage. Since you say you are using a memory mapped file, you may need to account for that in addition. For example, I believe the PageFileUsage
information returned from that API will not include information about your own memory mapped file.
If your application is monitoring the usage, you may be able to use FlushViewOfFile to proactively force data to disk from memory. There is also an API (EmptyWorkingSet) that I think attempts to write as many dirty pages to disk as possible, but that seems like it would very likely hurt performance of your own application significantly. Although, it could be useful in a situation where you know your application is going into some kind of idle state.
And, finally, one other API that might be useful is SetProcessWorkingSetSizeEx. You might consider using this API to give a hint on an upper limit for your application's working set size. This might help preserve more memory for other applications.
Edit: This is another obvious statement, but I forgot to mention it earlier. It also may not be practical for you, but it sounds like one of the best things you might do considering that you are running into 32-bit limitations is to build your application as 64-bit and run it on a 64-bit OS (and throw a little bit more memory at the machine).