That's interesting.

I've never seen one take less than about 6 hours. Also, did you remember to run the
W10 COMPATIBILITY TEST during the upgrade? A lot of people forget or don't know to do this and that takes extra time too. No one here on
WF has reported a time less than 6 hours for a W7->W10 upgrade, unless I missed something.

This also includes the Tech Preview versions too. Also, I've done a W8.1 upgrade to W10 and that takes the least amount of time, since there are many fewer updates from 8.1 going to 10 than from W8 or W7. I remember that one taking about 8-10 hrs. or so.
W10 COMPATIBILITY TEST
Get Windows 10 app - Check Compatibility Report for Windows 10
On your Dell laptop, how big was the used portion of your W7 drive? The size of your windows+data on the bootdrive makes a difference too. I've done upgrades on my test machine which have less than 20GB for bootdrive usage. Still took 6 hrs.+
Good luck with your upgrade!

And don't forget to make another backup image to external media, as a
HUGE W10 Anniversary Update is coming on
Aug. 2nd which is what Microsoft calls a "roll-up" update, which means it contains all the updates since Nov. 2015 last year. This is going to be the size of a Service Pack (e.g.: W7 SP1). We anticipate it causing problems on many machines. fyi.
BBJ