There doesn't seem to be any consistency to it and it's been happening for quite a while. My last couple have been instant. But have also had a 24hr wait recently. The amount doesn't seem to cause a problem but weekends seem to cause problems.
I had to deal with this incompetence in November. Tried SCB and was told OA wasn't a retirement visa 🙄
Went next door to Krungsri and opened an account straight away no problem. No requirement for any letters. Passport, visa, proof of address, phone number (so you can download their app) Done easy