Since 'upgrading' my Dell PC to Windows 10 Pro, the LT SPICE interface is excruciatingly slow, dragging a schematic has a 2 second lag. Analysis speed seems fine.
You could update the drivers with the Dell-update program. Maybe this helps.
I remember a Dell laptop where I could choose to use the CPU-graphic or the NVIDIA-graphic card when I started a program. Unfortunately I don't have this laptop anymore. Thus I can't check whether this option was possible for every program.