Mail is loading info extremely slowly

Happening more often. Mail isn't loading content. Shows that I have the email, but doesn't show content. I have to close Mail then reopen. Still slow. Computer is freezing also and "escape" doesn't work. Have to turn off from back button. I'm thinking I need to get a good Mac cleaner to resolve this. Any suggestions?

I have an iMac Retina 5K, 27", Late 2015 model.. 16GB 1867 DDR3, 4 GHz Quad-Core Intel core i7, 825 GB available of 2.12 TB