Hello@Gothman ,

Here is FAQ about the Powerline Rate and Actual speed of the Powerline Adapters which may help to explain better:

 https://www.tp-link.com/support/faq/2928/

tpPLC Utility shows the Powerline rate which is an indicator for the powerline environment, the actual speed is around 20% to 35% of the Powerline rate on the tpPLC Utility in actual household environment.  In your case the powerline rate on tpPLC Utility is 335 Mbps, if the actual speed is around 80Mbps, the speed should be reasonable.

You could check guide below and remove the factors that may affect the power line environment, it suggested trying a different wall outlet and see if that helps with the speed, 

https://www.tp-link.com/support/faq/409/

You may consider upgrade to 8 series powerline kit with MIMO feature to work with 4 series as FAQ here, 8 series may perform better, but note that real powerline rate is still closely to the real powerline quality/ environment. 

(How can I select the powerline device? )https://www.tp-link.com/support/faq/2725/