Solution: The classic Portable bug. Turn off "Code Hints" in ActionScript Preferences (Edit → Preferences → ActionScript → uncheck "Show code hints"). This immediately fixes the lag.
Solution: Windows may block write permissions for portable apps. Run Flash.exe as Administrator (right-click → Run as Administrator) just once to establish write permissions. macromedia flash 8 portable
Once you have obtained a legitimate portable package, follow these steps: Solution: The classic Portable bug
Before proceeding, an ethical and legal note: Macromedia Flash 8 is abandonware. Adobe no longer sells licenses for it, nor do they provide technical support. However, because Adobe still holds the copyright, downloading a pre-cracked portable version exists in a legal gray area. For educational or nostalgic use, many users proceed
For educational or nostalgic use, many users proceed anyway, understanding that no commerce is involved. However, if you intend to produce commercial work, consider using Adobe Animate (the modern descendant of Flash) which legally supports publishing Flash 8-era .swf files.
Museums restoring old kiosk interactives (circa 2006) use portable Flash 8 to edit proprietary .fla source files without virtualizing an entire Windows XP machine.