Halo 3 pc performance problem on a capable rig

As stated in the title, I’ve been facing this problem where my performance is lower than it should be, my system specs are quite good for this game but the problem remains.
My framerate can barely maintain 30 fps and it often drops as low as 15.
These are my hardware specifications:
CPU: Intel core i5-7200u
GPU: Nvidia Geforce 940mx 2GB DDR3 Vram
Memory: 8gb DDR4 Ram

This might seem like a below average pc which it is but I’ve run this game on an even lower end hardware which happened to be my cousins friends laptop.
He uses an i5 6th gen U series cpu, with a geforce 920m and 8gb ram and the game ran at 60-75 fps or so at 1600x900 resolution at enhanced graphics.

From what I’ve seen, this seems to be a software issue, and I’m not sure if it’s some rough installation or some of my drivers are messed up. But I’d surely appreciate some help. I’ve tried several things from Defragging my Drives, cleaning temp, disabling quite a bit of background processes, running the game at 720p at performance mode, tinkered with nvidia control panel, reinstalling my drivers.

I’ve changed the DPI settings in the compatibility options for the exe file and also made it run as adminstrator, which surprisingly boosted my performance to 90fps or so for like 10 minutes before dropping back to 15 fps.

EDIT: Forgot to mention that my nvidia driver version is at 451.67