My 820 acted really slow with a SDcard in it when I got it at launch, but with a update from Nokia a few weeks later it went away. Hopefully Nokia will release a bug fix update soon for any issues with the 720. I do notice a bit of slowing with my 720, but it's tolerable for me.
Sadly, after a week of using the Lumia 720 on a business trip, "tolerable" is about as good as I can say.
The SD card stuff was maddening for me. I had videos on the SD card, but the phone didn't show any under the Videos section -- even though they showed under the "Recently Added" section *AND* I could play them. Whatever. With the SD card inserted, photos typically took an extra few seconds to "snap" -- even though I had "save to phone" as the storage location of choice. The phone was also noticeably slower in various areas when the SD card was present. Lame.
So......I ended up removing the SD card when I wanted to use the phone in any semi-responsive way -- photos, etc. -- and then inserting the card again when I wanted to listen to music.
A *very* smart "smartphone" indeed.