You can use the Cloudflare DNS server to almost double the speed on your Firestick / TV. The easiest way to do this by changing your router DNS setting. Open the Admin panel of your router do to WAN setting and change the DNS Server Addresses to 188.8.131.52 and 184.108.40.206 click Apply or Save button at the bottom. Copy and keep your actual DNS setting, just in case this doesn’t work and you need to revert to your original setting.
I have tried this and It has helped me with my service provider. So I do encourage you to do the same. This also can be changed on Fire Stick TV, but it is very tedious using remote, hence I suggest you to this on the router level.