I have a few Latitudes at my home. (1 E6430, 2 E6320, 2 E6410)
They all have 8GB and SSDs.
They have all been upgraded to Windows 10 Pro and have been running great for months. Recently both E6410 have become extremely slow. Task manager doesn't show anything taking up resources. They are pretty much unusable. Since this has happened to both laptops at about the same time, I am thinking that a Windows upgrade caused this.
There are several things that come to mind. First off what drivers did you use when you installed windows 10? Windows 10 is not supported by Dell for the 6410. I ask because the drivers Microsoft would provide would be generic drivers and sometimes those fail out of the blue. Have you also trim'd your hard drive? You will want to go into the 'my computer' area where your hard drive is listed, right click on it, select properties and then optimize it (do this only if it is listed as a solid state drive). You did state that you looked in task manager but I wanted to clarify that there wasn't any unusual hard disk activity or processor activity? If it is a windows update the problem with that is it could be anything as microsoft now updates at random and forces the updates to everyone. Have you installed any programs that you can remember when this happened? When was the last time you checked for a virus or malware?