What you could do is reduce the maximum of graphic effects and colors.
1- Open a client generator, choose "Disable background and animation for better performances".
2- In the display tab, choose 15 bits colors.
If you are connecting using the HTML5 web client, open an admin tool \ web portal \ HTML5 client and check the "Use recommended value" box. Refresh your client browser.
Right click on "My computer" and click on properties. click on advanced system properties, performances, visual effects, and select "Adjust for best performances". This setting can be set on client and server side and will enhance graphic performance.
You can also make sure that your graphic adapter drivers are up to date as well as your RDP protocols and windows updates.
You can also force the use of the classic theme using group policy : To do so, type gpedit.msc in start search and hit Enter to open the Group PolicyEditor. Navigate to User Configuration > Administrative Templates > Control Panel > Personalization. Now in the right pane, double click on Force a specific visual style or force Windows Classic. Enable the group policy and leave the field blank to enforce windows classic theme. Reboot and open a command prompt and run the gpupdate /force command.