IMFSinkWriter very slow and use of MF_SINK_WRITER_DISABLE_THROTTLING

I have a test app using MPEG4 media sink and IMFSinkWriter. I am writing RGB32 samples. The sink writer appears to be incredibly slow.

Running the app the I can only process about 3 frames per second throught the WriteSample method and generating a 10s second video takes several minutes.

If I set MF_SINK_WRITER_DISABLE_THROTTLING to true the output file is generated very quickly - about 10 seconds. With a large output file WriteSample always returns S_OK but eventually I get E_OUTOFMEMORY from MFCreateSample which is fair enough.

My question is:

With a small output (250 frames) why is the SinkWriter so slow when throttling is enabled?

I would have expected that with throttling on or off it should take exactly the same amount of time to produce the output file.