Do CRT filters actually need to buffer an entire frame, or is that just commonplace because it's the easy way to do things when you're implementing them in soft

Do CRT filters actually need to buffer an entire frame, or is that just commonplace because it's the easy way to do things when you're implementing them in software? It seems like if FPGA based upscaling with CRT emulation is in scope, it shouldn't need to delay the signal by more than a few scanlines.