3rd retina display replacement is making my Mac crawl

I have just had the dead pixels on the Samsung retina display replaced with an LG LP154WT1-SJA2.

I had to have the original LG screen replaced one and a half days in to owning this Mac and now not even 18months old I have had to have the Samsung screen replaced because of dead pixels in the display.

Everything is running slower and I am even having Word just lock up on me.

I have 262GB free of the 512GB SSD hard drive so I don't think that is the problem.

Is there any setting that could have been invoked to give me these problems????

Is it possible to get the old screen back and get it running at normal speed??