First thing to verify is that the AC adapter is recognized.  Go into setup (F2 at powerup).  Is the adapter listed, or does it show unknown?  Note:  just because the system runs does not mean the adapter  powers the system does not mean it is recognized, and if it is not, the system will run slowly, as if on battery all the time. Second thing to do is separate the Windows possibilities from the hardware.  Restart, pressing F12 a few times at powerup.  Load and run the Dell diagnostics before Windows begins to load.  Do these hardware diagnostics complete without issue, or end in an error code such as 2000-0142?  
My laptop is slow.   Made sure I have the latest version of Support Assist.  The first step regarding downloads and drivers completes.  Several attempts have the Hardware Analysis reaching 90%, but no further.  I ran defrag after trying Support Assist.  Still hangs at 90%.   I have done soft and hard reboots.  Same result. always stops at 90% for up to an hour.   Any suggestions?  
Inspiron 5559 - Support Assist Hangs at 90% on Hardware Analysis