VirtualBox is very slow in Windows 10

I had VirtualBox installed on Windows 8.1 and it was working correctly. I upgraded to Windows 10 and re-installed VirtualBox, but it's very slow.

System Info:

CPU : Intel Core i5
Graphic Card : Nvidia 740m
6 Gigabyte RAM
256 GB SSD Storage

Drivers are up to date. 3D Acceleration is enabled.

What's the problem?

It seems you're on a laptop, so tweaking Windows power options may be a solution:

http://www.rawinfopages.com/tips/2015/05/speed-up-virtualbox-enormously-with-this-simple-tweak/

To quote:

So it turned out to be a simple problem, partly caused by selecting the wrong power plan. Make sure that the High power plan is selected when running virtual machines VirtualBox.

After some more experiments, I found that by raising the minimum processor speed when running on mains power raised the CPU speed. Setting it to 80% made the minimum speed 2GHz on mains power, which was fine for running the virtual machine.

To do this, go to Power Options in the Control Panel and next to the Power saver plan, click Change plan settings. On the next screen, click Change advanced power settings. Expand Processor power management, Minimum processor state, and set Plugged in to 80% or whatever value gives reasonable performance in the virtual machine.

I had the same problem today -- found that the "Graphics Controller" (Display tab) had defaulted to VBoxSVGA. Clicking over all my VMs, it was the only one on that setting. So I changed it to VBoxVGA, and voila - no more lag! It was so bad, I'd see the display update in the VirtualBox Manager mini preview, so I'd click off the VM then click back and the display would update. Good as new now.

In my case, it was because Hyper-V was enabled. I had it disabled in Windows Features but I was still getting the turtle icon in the bottom right of the VirtualBox guest window. To turn it off completely, I had to: