Inspiron 3646 pixelating videos

The first thing that comes to mind is memory.  Spec's say 8GB max and with what you are doing (and Win 10) if you don't have the full 8GB consider upgrading to 8GB.  However there is only one memory slot so if you don't have 8GB and want to upgrade it will require a compatible 8GB memory module.   

You can also run the free Resplendence Latency Mon.  Although it is for audio, it may give you some insight for video too.

Finally, and being direct, considering hardware advances since 2014 when this model was introduced it may time to consider a new PC.  This was a relative "entry level" PC and with what you are doing more than an "entry level" computer is really needed. With only one memory slot the CPU can only run one channel processing and not dual channel as most, even in 2014 were configured.  Win 10, as the OS is updated, has dropped or limited support for older hardware and a 2014 vintage machine has "older hardware".

• Do these same vids play "normally" on some other PC?
• What size and how full is your hard drive?
• When was last time you backed up your vids on an external USB drive and deleted them and any garbage files from the internal hard drive to free up space?
• When was last time you de-fragmented the hard drive after backing up and deleting some files? (Don't do this if it's an SSD).
• How much junk is loading at boot and running in the background?
• Have you scanned thoroughly for malware recently?

These play normally in VLC Media Player, which is also on the machine.

Hard drive 500 GB, but videos are playing from external data book.

No need to back them up as they are on an extyernal HD.

There is quite a bit loading at start up as I a a developer in my daylife.

Drive has been scanned by McAfee for vulnerabilties.

I have upgraded the memory to 8 GB because I was having some ;latency issues.