Never used DXtory, but I have Fraps (paid) and MSI AfterBurner (free). You will see some frame rate drops, especially if you use the frame lock feature to keep game and video at 30fps for quality. But I've not seen it drop to 2.
What are you recording to? SSD? Is TRIM enabled? Is Indexing off for this drive? Is page file off for this drive? Are you using the record drive for anything other than recording? What's your CPU load when recording? GPU load when recording?