So, It is viable to create a javascript emulator that will run at full speed? The answer is yes.

Results greater than 100% mean that will work faster than on a real PSP (in theory):
http://jspspemu.com/#samples/compilerPerf.elf

Due to javascript to not have “goto” keyword, it will harder to make it faster, but it is definetively possible to get fullspeed with current hardware.

http://jspspemu.com/#samples/sprite.pbp

About rendering speed. jspspemu is using WebGL and the speed is mostly limited by the gpu, so the rendering speed will be similar to other emulators using OpenGL ES.