Some quick insights. Bank transactions typically don't process on the weekend, so if you make a payment on the weekend, it might not actually clear until, Tuesday. It will process on Monday and maybe clear on Tuesday morning.
The banks probably do this, to prevent fraud as well. If it was instant, than that opens up the door for fraud. Ever hear of check kiting?
Also, I left article here, to better understand how it works.
Why isn't my credit card payment instant?
"The account you use to pay your credit card affects when your payment posts. Your payment typically posts fastest when you pay with an account from the same bank as your credit card. Payments made from an account with another bank take longer to post."
Why does it take 3 days for a credit card payment to go through?
"Credit Card Transactions: Typically take up to 3 business days to clear.
Credit card payments often involve multiple layers of verification and authorization, contributing to this timeframe. Direct Deposits: Usually clear by the next business day."
Why hasn't my credit card payment posted?
"It can take one to three business days for an online or phone payment to post to your credit card account and reflect in your available credit. 1 That's because
payments made using a checking account and routing number are processed in batches overnight and not in real-time."
Why do payments take 3 to 5 days?
"The reason a BACS payment typically takes 3 days to clear is that
there's a cycle that posits the company submits the payment on the first day, the payment is delivered to the bank of the recipient on the second day and the payment is settled on the third day."
https://financeband.com/why-don-t-credit-card-payments-post-immediately