On my Allwinner H3-based miniPC enabling the NTSC filters slows down the emulation too much, also the NTSC shader is too heavy for the Mali GPU. Which alternative do you suggest me?
No alternatives exist.
For weak devices I tend to use the phospor shader setting it to 2x, it gives me a nice look very similar to NTSC, also another cheap option is simply use scanlines or an overlay 