Additional testing reveals that writing really large files must simply be more likely to reproduce the problem, but in fact is not a requirement. With a NTFS partition near capacity (around 4 GB remaining), I was able to reproduce the problem by writing a file that is just under a GB to it. When I noticed that it slowed way down several seconds into writing the file, I aborted and freed an additional half a GB more (giving a total of ~4.5 GB free). I was then able to write my file not-so-large file at normal speeds. So it seems this may be due to some odd fragmentation corner case.
Additional testing reveals that writing really large files must simply be more likely to reproduce the problem, but in fact is not a requirement. With a NTFS partition near capacity (around 4 GB remaining), I was able to reproduce the problem by writing a file that is just under a GB to it. When I noticed that it slowed way down several seconds into writing the file, I aborted and freed an additional half a GB more (giving a total of ~4.5 GB free). I was then able to write my file not-so-large file at normal speeds. So it seems this may be due to some odd fragmentation corner case.