The space thing is because internal storage and SD are different. By default apps install only to internal storage, and their data also live there. SD is really for media storage (it's both slower and less reliable than internal storage), so while it's useful to have it isn't really a substitute for having a decent amount of internal storage. And when you use the internal storage up you will get that message no matter how much space is on SD.
So, what can you do about it? First, if you are currently keeping media on the internal storage, move them to SD. Note that "/sdcard" is really internal storage - I don't know what your phone will call the real SD card, might be sdcard1, or extsd, but /sdcard and sdcard0 are internal storage pretending to be SD (it's historical, comes from Google not wanting to break old, badly-coded apps, and frankly I think they should have just forced lazy developers to update rather than adopt these confusing names). Your camera app should have a setting to let you choose where it stores things, and the file browser (My Files I think Samsung call theirs) should let you move other stuff.
Secondly, there is an option to partially move apps to SD. Go into settings, app manager, downloaded, select an app and see whether you have a button to move it to SD. You can't move pre-installed apps, and some user-installed apps won't let you either (it's up to the developer), but this will free some internal space by moving parts of the app to SD (not app data, but as SD is slower that wouldn't be ideal anyway).
Finally, if there are pre-installed apps you don't use (make sure you know they aren't needed) which have had updates or saved data you can free some space by clearing their data, uninstalling updates to them and disabling them. You can do this by going to settings, app manager, all, selecting the app and then using the buttons there. If you don't know what an app does don't just assume you don't need it (it should stop you disabling really crucial things, but you could delete data you might not have wanted to).
Freezing might be if you are really short of space. Slow is possibly limits of the hardware, though with more space and maybe some junk disabled it may improve. For the charger, try another cable - I've had experience of defective cables causing phones to charge very slowly even from high power chargers.