Yup, that’s Slate. Even having separate tabs in the main window that aren’t rendering anything causes a performance hit. UE4 and 5.
But if you want to PIE without too much performance penalty you can minimize the editor after opening PIE in a new window or just use Standalone.
Runtime DataTable : Import and export game data to and from CSV or Google Sheets while your game is running!
easyCSV : Read data from any CSV and put them in an easy-to-access string map!
iTween : Free, smooth, procedural object animation