Different casinos offer different versions of casino games created by different software companies, so the payouts on each slot machine do not necessarily have to be the same from site to site.
Casinos also have the option (within a range) to determine themselves what percentage they pay out. You can always check a game's payouts in the slot game rules section.
What affects the actual payout speed, the time it takes from a withdrawal request to the moment it reaches your bank account, is determined by the administration of the casino. The terms and conditions state how long this takes. At casinos with a Dutch license, the money will be in your account within a very short time.