My download speed is staying at around 300-400 kb/s even tough I capped it much higher. My internet is very powerful so I don't think it should be a problem.

It also depends upon what you're downloading, and where you're downloading it from.

Small files may not hit max speed before they are completed. Larger files will usually get up to a nice speed over time.

Also, the server(s0 in question make a huge difference. They may be capped in what they offer to the public, they may not support download resuming / multiple connections, or they may even be bottlenecked because of an ongoing issue where they are located.

So, until you do more research, no way to know what the cause it.