RIFE TensorRT is faster than ncnn/Vulkan. I use MPV. The last time I tried MPC with my settings the video was choppy and I didn't bother investigating since I didn't need it and MPV works for me. I don't think the Automatic setting holds a candle to RIFE in reference to image quality. That being said the RIFE model you use will effect how good you're image quality and motion is. I use TensorRT, 2 GPU threads. I opted for AI model 4.16_lite because the image clarity and motion seems to be the best for me. I run 4k or less video but I have a 1080 display. I run Fixed 120 FPS - though this frame rate is only achievable for me by decreasing the frame size.
The idea is to choose a FPS you want then decrease the frame size until that the GPU can handle that FPS smoothly. Decreasing the frame size decreases the image quality. If you have a 4K video but you change the frame size to 1080 so it runs smoothly, the output image quality on the screen will be 1080 (shrunk from 4k.) For me to hit 120 FPS smoothly, I settled on 1920x1080 minus 22% which is 1497x842. So if I loaded a video that has one of the dimensions larger than that, it will resize it appropriately - most any video I load runs fine. I had one youtube video that made me enable performance boost in RIFE because with it I was just over the line of what the GPU could handle and it ran slow. I could of also lowered the frame size a bit more but I didn't want to or have to.
To enter a frame size, hit the 3 vertical dots on the top right of the main window and select Application Settings. When the Utilities window pops up, enter Size in the search box on the bottom right of it. Change the value for resize and make sure a "-" is right before the number. This is an example resolution value: -14970842
Notice there is no X in the resolution and for the height, a 0 is in place of the 1 when the number is below 1000.
Example:
-19201080
-14970842
I knew my GPU couldn't handle 1080 at 120 fps I subtracted 15% from 1920 and 1080 then kept going until it ran smoothly. With a 5080, you should be able to run the same fps at a higher frame size than me. You'll just have to test how far below 4k you are willing to go for your desired FPS.
I run with Scene Change Detection disabled for better smoothness (implying the frame size is set correctly for my FPS.)