Most of the time mine is sitting at 0% CPU usage and 160MB memory. When it does use the CPU it does not seem to go above 1.7% though I am sure it has and I just haven’t observed that. Certainly doubt it has gone as high as 50% and never continuously. That is on Windows 10 build 18363.752, so fully updated with the latest .NET. I am also using the latest eM Client beta, and the database is on an M.2 SSD. You can download an earlier beta from the Release History if you want to give it a try.
But with your current installation, if you go to Menu > Tools > Operations, is there something going on?
Maybe start that version of eM Client with a blank database and see if there is any difference. To do that close eM Client, then rename the C:\Users\yourusername\AppData\Roaming\eM Client\ directory. You may have to enable hidden items in Windows Explorer to get there. After restarting eM Client, add your email account again and see what happens.