I had a shield tv for a couple of months. For emulation, I noticed quite a few little glitches while replaying the original zelda. I didn't notice much lag while playing (but it was there, as I found out). I bought original consoles and flash carts to test. That is when i noticed how much lag there actually was on the shield. On the NES, controller input is faster than I remembered. I haven't played enough to find out if all the glitches I noticed on the shield were on the original (moved, and just haven't had time. I ended up returning the shield to get the new version. I cancelled that preorder when I got my PS4, so I haven't further tested the emulation. I don't think I will even though I plan to buy another shield for the other TV. As a media player, it is worth the cost. The PS4 is just as good (except for kodi and emulators), but both are way better than the rokus, apple tv, and fire. It is not even close.
I have the original consoles now, but I am looking forward to trying the virtual console on the switch when it comes out. If it isn't laggy, I will probably play on that instead. If I can find one, I may try the SNES classic, but I would rather play on the switch. To be honest, I am enjoying the indie games on the switch so much, that I don't think I will have much time to emulate old stuff. Between the switch, and PSVR, my gaming needs are pretty much taken care of these days.
I normally gloss over Howard's Nintendo praise, but the Switch really does live up to the hype. I game more with it. I like playing on the tv, and as a portable. I have owned it for just short of 4 months, and I have purchased 14 games for it. Just being able to pick it up and play for a few minutes, turn it off when you run out of time, and pick it up and pick up where you left off makes it the most useful console I have ever owned.
Go buy a shield though. It is a great device. I hope it does really well. Lowest price shouldn't always be the winner for these devices. This thing is way ahead of the competition, and I think it is well worth $200. I don't think there is a better emulation machine out there, but for me personally, that is just a side use for it.