Does using a slower card in the second slot slow the buffer clearing?

It will if you've got the camera configured to write to both slots (Backup or RAW+Jpeg). If you're set on Overflow, buffer clearing shouldn't slow until you start saving images to the second card. Personally, I prefer identical cards in both slots: same size, same performance, same vendor. There have been too many issues with subtle card differences in digital cameras over the years, and we still see them too often in the latest firmware to risk encountering one. So I test cards in a camera to verify that they perform the way I want and without incident, then use matching ones in both slots. Even then I've seen one instance where a card failed (at which point I had to replace both cards to get the camera back to normal). Cards are relatively cheap; don't skimp.  

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