Very strange. Not sure other then the usual,
- Is your router lanned to your pc?
- did you change hardware? Ie: laptops tend to suck at streaming.
- If youre on wifi, do you run your computer on 5g versus 2.4 that your 3ds is running on? If so change both to 2.4
- Move closer to router
- turn off and on your computer and 3ds a few times (randomly youll get a shite connection for some reason)
Other then that im not sure - if you were getting good performance before its quite strange
I'm running it on my laptop and I got decent framerates before that varied every now and then from 20-30.
I have a 802.11b adapter, and my router broadcasts 2.4.
My router is literally right next to me.
I'll try restarting a few times.
EDIT: I think it might have something to do with my router. I tried Monster Hunter XX with my Japanese n3ds, and I got the same result. Smash Bros plummeted to 7fps at one point, but was still very unwatchable.