Apex Legends high Input lag tied with fps

so I've been having this since the start of season 9, or maybe since the split. I can't remember but this has been going for ages. 

I realised it only affects the mouse as input, not controller. I can play fine for hours with the controller and there is 0 input lag. It actually feels a lot better than playing on the Xbox for me since I can play at 120+fps while Xbox caps framerate at 60. While using the mouse, on the other hand, I can feel a delay at many random times during a match, sometimes a very strong delay. The waterfall is the most obvious place it happens, I have to actually avoid fights in that area as the frame drops dramatically there and the input lag is horrible. What is interesting is that even though the frame drops in locations such as the waterfall, it is not actually low enough to supposedly impact the game, I'm talking 80-90 fps, instead of the usual 120-150fps. It's no reason to make my game so sluggish and increase input delay. Also, there is 0 input lag on the controller even at the waterfall. I can play perfectly fine. 

Adding to the above, I thought it could be related to using a wireless mouse, and maybe the high use of GPU or CPU was impacting the wireless dongle functionality, but I tried a wired mouse and also a wireless controller using the same USB port. Same results: 0 delay with controller, but delay with mouse.