To fix a slow Samsung Galaxy phone, restart your device to clear temporary files, clear cached data in "Device Care," keep your software updated, close unused apps, disable auto-sync, clear individual app caches, optimize performance, reduce animation scale in "Developer options," use simple wallpapers/themes, and consider a factory reset as a last resort.
Ensure to back up your data before resetting your phone. If the problem persists, contact Samsung support or visit an authorized service center.