Chrome lags in Windowed mode

Hi, I’m playing Hearthstone on a brand new iMac and have noticed a few issues already.

First thing, I was getting a glitchy, horizontal blurry line (if that makes sense) across the middle of the screen occasionally, usually noticed when cards were drawn at the beginning of some turns. I changed the in-game “Quality” to High and that seemed to eliminate it, but now I’m noticing that when I have Chrome browser open and Hearthstone in windowed mode, and I bring Chrome to the forefront with Hearthstone behind it, Chrome becomes really laggy. Just curious if anyone has experienced this and has any tips. Thank you.

iMac (Retina 5K, 27-inch, 2020); 3.8 GHz 8-Core Intel Core i7; 8 GB 2667 MHz DDR4; AMD Radeon Pro 5500 XT 8 GB