Today, Microsoft's Garbiel Aul, who's leading the Windows 10 Insider program, said that he's currently testing Windows 10 a Nokia Lumia 635, and the OS is "very smooth on it overall." As you may know, the Lumia 635 features 512 MB of RAM and a Snapdragon 400 processor, so it's one of the least powerful Windows Phone 8 devices out there. If Windows 10 runs smoothly on the 635, it will most probably also play well with handsets like the Lumia 530, Lumia 525, or even the Lumia 520.
A Windows 10 preview for phones should be launched pretty soon, and handsets with 512 MB of RAM will get it, too.
As for the final and stable version of Windows 10, this is expected to be available starting this summer. That's not exactly soon, but it's probably worth the wait.
source: Twitter via WMPowerUser