Who need design. The site should load and work smoothly.
I doubt someone wants to play on a lagging platform, where a button jams and nothing loads.
Simple, clean.
many users ofcourse look at the design too, if the design is quite low quality then most users wont visit a site again even it has good site speed and features
Well, if your design is hyper fancy like on dadice for example, it hovers and slides away on smaller screen, the features wouldn't care much, as for the annoying ping at 3 am, that the SSL is down.
Same goes to Primedice good light colored design which is really nice, but the fact there is a lot of people, using autobet and canceling it you have to wait 15 seconds or more, from time to time the button roll jams.
I still take platform over design, for example bitdice, simple, dark and haven't had lag and loading issues (exept nginx from ddos).
If you tell me that my internet connection is poor. I got 10MB/s up and 40MB/s down.
I am pretty sure most people visit sites pretty sites because they have faucet. People that gamble with real money, they go where things work.