mac book pro runs slow when hdmi cable is attached

After reading hundreds of the same complaint as mine with no solutions at all, I can only assume the designers at mac have done this on purpose so you will buy their own 27 inch monitor for thousands of dollars more . I bought a new hp monitor 27 inch for $199 from office mac for my mac-book pro and attached three different hdmi cords and the computer is useless and goes so slow its shocking. I tried the new mac book pro 2018 and the older 2015 and the same problems occur. Mac support offers NO help for this at all. There maybe lots of geek heads who think they no the problem but it always comes back to a useless computer. As soon as i unplug the hdmi from the laptop works fine.....