Hello,
The thumbnails generation is very complex process now. Sadly the API that was available before for the SpeedDial is not available anymore.
There are three options for generating thumbnails and each of them has some advantages and some disadvantages.

But the best results are from the background reload - which works in the invisible tab. These thumbnails should be sharp.
This method is by default used when you add a new dial (unless you use "Add this page to dial" from context menu on a page).

You can also use reload by visiting page which also yields good thumbnails and it works on any page.

I will try to simplify the options in near future for easier usage...
Also feel free to contact me directly on my support e-mail.

Best regards,
Juraj Mäsiar