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5 Wilds, payout for 5 scatters is fixed to 500x no matter how many lines you play except you hit 5 scatters on free spins as it gives you 1500x. 5 Wilds gives different payout/multiplier based on how many lines you play. The less lines you play, the higher payout you'll get if you hit it (max 10,000x on 1 line). Talking about the fast/instant spin mode, I prefer not to use it because it is just too fast so it gives less sensation while playing the game.
Is this Scarab slot is a slot that originally made by Stake? I haven't played any original slot by any casino because I think that those slots are less likely to allow us winning a lot. I didn't thought some of them might be one of the best slots for other users. I'm not very old in playing slot, in fact I just played slot months ago, and I honestly do not know any strategy how to play slots that pays per line. I tried some, but I'm unfortunate, maybe because I really don't understand well those types of slots.
I do not even know if they are provably fair or not, I really have no idea, ~
Stake originals
are provably fair. What you need to do to check the fairness of the outcome is to select "My Bets" here

Then, now, this is important, you have to click this

and then here

you click "Change". After that you go again to your game in "My Btes", click on it, and now, since the previously hashed Server Seed is revealed, you can click this

at the very bottom of the popped-up window, and check whether the picture there is identical to what you got with your roll.
Next, you can also click

and read more about the process of generating the outcome of your roll.