Is citra really slow ATM

I am playing NSMB 2 and Link Between Worlds and it gives me 20 - 30 fps and its stutterring sometimes.

i3 4010U @1.7Ghz

can this spec run any games on citra at playable speed?

Idk about your specs but 20 - 30 fps is quite ok

is citra using single thread? will it change if they go dual core?

Yes, Citra is only using a single core, however, multi-core won’t change anything. In fact, the 3DS also uses only 1 core for games. A dev tried to do a multi-core test in Citra, but it failed, because the games ran even slower than the latest Nightly at the time.

Your laptop isn’t strong, so don’t expect to get full speed emulation even in the future for a lot requiring games. Low power CPUs in laptops are pretty bad for emulation.