ParaLLEl-RDP is not actually related to Angrylion at all. The author compared ParaLLEl-RDP’s output against Angrylion’s, pixel by pixel, to ensure correctness, but that’s the only real connection.
ParaLLEl-RDP is a software-rendering plugin that uses Vulkan compute shaders to do the software calculations massively in parallel (hence the name). Sort of like using CUDA or OpenCL, if you’re familiar with those technologies, but all inside of shaders instead of using those specific languages and frameworks. The big drawback is that it requires some advanced Vulkan extensions, which some GPU vendors neglect to support in their drivers (when you run into this, it’ll usually print a big ERR in block letters on the screen).
ParaLLEl-RSP is a highly accurate, LLE dynarec for the RSP. It is comparable to cxd4, but since it’s a dynarec, it runs much faster, similar to the CPU dynarec vs interpreter.
Threaded angrylion runs entirely on the CPU, which makes it work on more hardware, but it’s slower/more demanding, even with the threading (the single-threaded version is unusably slow even on modern CPUs). It also doesn’t support any fancy stuff, like increased internal resolution.
So, with those things in mind, ParaLLEl-RDP plus ParaLLEl-RSP is going to give you the fastest LLE/accurate setup with the added ability to upscale. The drawback is you need advanced Vulkan support and good drivers to run it.