There can be multiple things at play but one thought it’s you may be hardware constrained. This is not limited to just your server but also how you are connecting. If you are using a budget cell phone with a bunch of things installed and running it will get slower. If you use a bunch of social media apps disable them before starting a test so they are not running in the background or eating into your connection.

Another is it sounds like you are using your Tailscale connection to then reach out through your regular connection to a speedtest. So you’re using the same connection with multiple data streams at the same time. You will be limited to your slowest speed when sending data to or from a server through you tailscale connection to the internet so give up on that 109Mb/s. You also have to take into account what your connecting devices speed is, if you are on cellular with an 18Mb/s connection that is your top speed.

Finally instead of running a speed test to the internet, spin up an instance of openspeedtest on a computer that is not hosting your tailscale connection and test to there. When you are starting to setup a homelab it can be useful to have an in house speedtest anyway.