Could you share what MacOS version and XCode version you are on?
UE4.23 uses Xcode 10.3
UE4.24 uses Xcode 11.x
These need to fit together.
Might be light calulating and/or shaders compiling during runtime that causes the slowdowns.
Did you “Build” before play?
Also in the editor try “File > Validate Data” before you hit play, this will cause shaders to compile. It will include editor content if in the content browser “Show engine content” is on.
Unfortunatly this will take a while, so better let it run through after hours.