The Lenovo ThinkPad X61 is the small laptop that just keeps going. I have installed multiple versions of Windows on the Lenovo ThinkPad X61 and Windows 10 does not disappoint.
For this test I did a clean install of Windows 10 Pro X64 update 1511. I did the install using an external DVD drive but you could probably use USB flash drive as well. After the install I ran Windows Update (like you always should), and after that there were no more drivers to install. Making this an easy setup.
Now while Windows 10 is a free upgrade to Windows 7 and Windows 8.1 it is not to Windows Vista which came on this machine. If you have an open license of Windows 7 or Windows 8.1 you can use that key to install and activate Windows 10 on the Lenovo ThinkPad X61. Below is the Device Manager and System Properties.
As far as performance, I do not have any numbers to compare it to but Windows 10 feels just as responsive if not more so than Windows 8.1 or Windows 7 did on this machine.
If you have one of these Lenovo ThinkPad X61 Windows 10 is an obvious upgrade whether you are running one of these as a personal computer or as part of an enterprise.