No one has mentioned the most likely issue, i.e. a version of Windows 10 that was put on the laptop at manufacture surely has some/many Windows 10 updates to install that will make the device seem creepy/crawly. It could be as some others have suggested but Win 10 updates in the queue are certain.
I use StartIsBack and have tried Classic Shell and neither slows down Win 10. I have StartIsBack on 4 Win 10 PCs with no problem and a more usable (in my opinion) interface. And all of these PCs have had multiple Windows updates in the last couple of months.