Tax cheques are at the banks discression, they can clear them instantly, however are not required to. For example the $900 stimulus cheques the governement requested that all banks clear those cheques instantly (guess they wanted the money bank into the economie right away), however with TAX cheques the banks arent require to, but sometimes do.
If you deposit your cheque at a branch its going to take 3business days to clear,
If you deposit it at a POST OFFICE, it will take 5 days to clear.
(the reason not all tax cheques are cleared instantly, as they are one of the most fraudulent cheques - as dodgy people know that they have a higher likelyhood of being cleared instantly, therefore open an account they dont plan to use, deposit a fake cheque in there, take the money out, and then the bank wont know about it for 3days!!!
Naughty people ruin it for all!!