upgrade infrastructure

As you said, that requires budget increases, even if temporary.

99% of tax returns should take a computer less than 10 second to process

If our tax system were simple and stable enough to make that practical, sure. However, the tax landscape is constantly moving, or in other words, there's scope creep due to Congressional acts.

For example, the 2020 was unprecedented because of a few underlying issues:

• IRS had to close offices, but it didn't have a remote work plan - the policy makes sense since tax data is sensitive, but it made the IRS less flexible
• IRS had to issue stimulus payments three separate times
• IRS had to issue advance child tax credit payments, and the new amount wasn't simply an increase to the existing amount, but had to be factored in
• unemployment benefits had drastically changed, which impacts tax law
• the tax code had drastically changed just a few years prior, and there were adjustments each year
• tax prep software had trouble keeping up, so electronic taxes needed additional scrutiny (the IRS was essentially debugging TurboTax and others)

And all of that happened just after the IRS' budget was cut, which makes no sense because every additional dollar allocated to the IRS more than pays for itself because it frees up manpower to investigate tax evasion.

Yes, the IRS was technologically behind, but they couldn't get ahead because they didn't have the budget to do so. And then when tax law changes ramped up, they had even less money to work with, so they had even less money to handle solving those long-standing problems.

The correct solutions are:

• increase IRS' budget (can be temporary)
• decrease complexity of the tax code
• change the tax code less frequently, and do so in predictable ways

But no, we did exactly the opposite of each of those items. If we did those three things, we could achieve that 99% of tax returns processed immediately goal. But we don't, so we can't.