OpenPandora isn't exactly a popular platform... As such it doesn't have the best coders available. I've seen the PSP n64 emu running better than it, and OP is much more powerful than the PSP
The 3DS should be more than powerful enough for emulating the N64 on SOME level... What level would be up for debate since we don't know much about the 3ds... We only know that it uses a custom dual core ARM processor with an integrated PICA200 graphics chip running at who knows what speed. We don't even know what speed the processor runs at and won't have any idea what-so-ever till the system is truly hacked OR if some verifiable internal nintendo documents get released.
The latency may be too high from the slot-1 carts to the 3DS to be able to play all the games outright, the majority of N64 games should be able to be loaded into memory so that wouldn't be an issue, and the latency from the SD slot SHOULD be fast enough to handle the larger games... That said, if we have a native 3DS slot 1 card, that may have plenty of bandwidth too... They are supposed to have better latency/bandwidth than regular DS/i... but that's another big question mark.