Your PC could have bloatware installed that's slowing down the system and hogging resources. Another potential cause is outdated drivers, such as the graphics driver, which can sometimes cause performance issues. Your PC might also be overheating, causing the system to reduce the speed of the CPU and GPU to prevent damage, which would lead to performance drops. Poorly coded browser extensions as already mentioned can also consume a lot of resources, causing slowdowns. It's also possible that there are other applications or background processes running that are consuming resources and slowing down the browser. Lastly, a significant cause of slowdowns could be that your 128 GB SSD is nearly full. If the drive is at its capacity, it can slow down the system significantly.
Troubleshooting your system may involve a range of diagnostic steps. These include, but are not limited to, removing all browser extensions to test for conflicts, verifying your graphics card driver version against your system information, and utilizing a third-party application (Core Temp, Monitor, etc.) to monitor PC temperatures during buffering events. Opening Task Manager with your browser running to identify which applications are consuming system resources.