You still have the option (for now, anyway) of flashing your phone back to Windows Phone 8.1, but you’ll need the special Windows Phone Recovery Tool to do so. Note that this will only recover the phone to vanilla Windows Phone 8.1, so you’ll probably want to update it to Windows Phone 8.1 Update 1 afterwards.
Credit to Microsoft for ensuring the Windows 10 experience on older hardware is snappy and responsive. The bad old days of laggy Windows 10 preview builds seem to be over. I haven’t noticed any “loading...” screens while running apps or flipping back and forth to the home screen.
Now that you’ve upgraded to Windows 10 Mobile, you’re part of a rather exclusive club. Let us know how it's working for you in the comments.
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