I don't understand why my computer is having such a hard time running TF2 on nice settings. I'm forced to play in in DX8 if I want the smoothest experience. I can have max graphics in maps like MGE or jump maps, but whenever I even have mid-high settings in a populated server... I get input lag and just a horribly, choppy experience. This isn't a problem for Overwatch, as I can run it on Epic presets (the highest) without lag. The same goes for the majority of games. I understand TF2 is poorly-optimised, but it's my favourite game and I want it looking beautiful.

Intel(R) Core(TM) i3-6100 CPU @ 3.70GHz
Radeon (TM) RX 470 Graphics

My little brother (I know) has a PC which cost him £200 less than mine, yet his TF2 runs well on very nice settings in proper matches. He even has the same CPU as me. He does have Nvidia, though. Is my graphics card good enough? Do I need to upgrade to a high-end i5? I'd really appreciate some help.