Online high roller casinos are the gaming platforms where players can make the payments (while depositing or withdrawing) with help of different methods as bank transfer, credit or debit cards, e-wallets and etc. The founding idea of gambling sites for whales is to create a convenient and quality system of payment that is recognized and intuitively clear to all customers in order to improve the options quality and general performance of clubs and at the same time enable them to satisfy all preferences of their clients. Nowadays, gamblers have the opportunity to make payments not only with fiat currency as euro or dollars, but with cryptocurrency as well (e.g. bitcoin). All fees and transactions time depend on the bank and other services. High roller casinos just provide the secure operation flow and safety of personal & financial data. Sometimes they can ask for a verification of the currency source, but these cases are rare.

More detailed info about banking security in high roller casinos

All trustful gambling platforms should be approved by eCogra. It means that the secure connections with the cashier are given;
Most high roller casinos use 128 or 256 bit encryption systems which are highly secure and the same as used by the web banks