One problem is that if you cannot keep up, adding in
all-zeros data will just make it harder to keep up. In
general, modern PCs should be able to keep up with 25 MS/s
without problem unless you are doing a lot of processing. We
are actually able to keep up with 300 MS/s on the X300. So
the question is more about why the app can't keep up.
An alternative might be to stream to a file. That should
keep up without dropping as long as you have a fast drive.
Then you can process samples from that file at the pace your
app can keep up with.