The prosecutor has been going over literally tens of thousands of pages of evidence to build a case. The defense is also entitled to go over many of the same documents. Short of hiring a literal army of lawyers, there is no way to speed this up. Even hiring an army doesn't solve everything. Communication channels increase geometrically with the size of the team, and past a certain point, it slows things down more than it helps. Worse, combinations of things can be missed by two different people seeing two different documents that together would point to something, but it's never adequately communicated across the team.

The trial you speak of is the end result of months to years of this process. It typically takes 12-18 months for a federal prosecution to get to that point. Even that is after they've been gathering evidence for some time before that. Trump's case is nothing unusual in that regard.

On top of that, federal judges have an oversized case load. We could probably quadruple the size of the federal bench to get it to something reasonable. Which means there's a very good reason to expand the bench beyond unfucking the fact that Trump stuffed it after McConnell held a bunch of seats open under Obama.

The one thing that is uniquely slowing it down is the Supreme Court taking up the presidential immunity challenge. The other federal trials are on hold until they make some kind of decision. That wouldn't necessarily mean a full hearing of the Supreme Court, or if it does get that far, they may undo the stay that's currently stopping trials from proceeding. If so, that would be an indication that they don't think Trump has immunity, but want to put their stamp on a constitutional issue that hasn't come before the court before.

Otherwise, this is how the system works for everyone. It needs to be fixed in general, but Trump is not getting any special treatment. This length of time is far from unusual.