How many people are playing at a casino and making a profit in the long run? The answer is very straightforward: very few. So when a casino offers instant deposits but withdrawals take many hours, claiming it's manually checking for cheaters, it's rather strange, since most people who deposit lose everything. Few people manage to win big in any section, so I don't see why they would manually check each person's withdrawals.
They could just offer instant withdrawals and only monitor those who win consistently and large amounts. In that case, the casino could investigate the person's account, but while the person would withdraw their winnings instantly, if cheating activity was detected, then the next time they won using cheating, they would freeze all the person's funds and block their account.
For Spinly, as I posted before in this thread, a Spinly representative said that they release the withdrawals instantly. However, if your withdrawal is tied to some kind of freebies, such as bonuses and free spins, then it will be handled manually. The other withdrawals, which were initiated without any bonuses, would be released instantly. I am not sure if you are one of their player and faced withdrawal delays. If that happened, you should share your experience, and we could ask a Spinly representative to share their feedback about the delayed withdrawal.
However, they also mentioned that the earnings from the signature campaign are considered a bonus, which is why it is also handled manually. However, the signature participants saying it does not take more than 24 hours.