It is FPGA based, due to this it can be configured on hardware level to exactly replicate the original hardware of the retro system. This and that it runs directly and not through some emulation layer and modern OS and stuff means that it gets as close as original as it can be, with zero lag and delay.
Not only perceive, it is often multiple frames from multiple lag sources (input lag of the USB controller or even worse Bluetooth, display lag from the monitor, rendering lag from the emulator, framebuffer lag). Playing fast paced games with frame perfect movement (Megaman on the NES for example) is so much harder on a emulator with all the lag, even on very recent hardware.