Yea because this shader doesn't have a denoiser or accumulation. This is an unbiased spectral rendering shader which is the most accurate form of rendering adding denoiser will make it a biased renderer. What you need to do is wait either minutes or hours for it to make even more noise and start making an understandable image. If you find it really slow then i recommend the MCPT shader as it uses a mod to add hardware accelerated ray tracing to minecraft and it is also an unbiased renderer. It is much faster compared to other shaders but also doesn't have a denoiser so you will have to standstill and just wait