Your bank does not have your money because it has lent it to someone else. If a bank has 5 business days to make a transfer then it can buy bonds, put money into an overnight deposit and do many other things. If a bank has 10 seconds then it has to maintain larger reserves which is a pile of money doing nothing.

A good bank would model cash flows to predict capital requirements and borrow from the CB as needed. Instant payments increase bank’s capital requirements and reduce margins. An EU bank cannot borrow from US Fed Reserve. Therefore EUR -> USD transfers are more problematic. The bank has to keep some USD buffer and/or be able to borrow in USD. Lenders will do fraud checks at various steps in the process, some steps can be manual. It does not happen at the time of the instant payment but rather triggered by it at a later time. Still, the end result is bank needs more capital.

As a user of fiat financial system you would observe: 1. Cross border instant payments are rare. 2. Large instant payments (>1M) do not exist. 3. Large payments may stuck at a middleman. Especially true at the time of political instability, even more so between unfriendly countries (e.g. China - India).