Brightline's original plan, when it was a subsidiary of FEC (whose r/w and track it's using in part) for the Florida run was top speed 125 mph, with the urban segments in coastal Florida (with numerous grade crossings) at the conventional 79 mph limit. The 125 mph top speed made it eligible for US DOT HSR financing, but that was about the only group that called it "high speed" though to be fair it *is* high speed for the US, and matches the top speed of the Northeast Regional trains.

Desert XPress/XpressWest's original plan, that it got NEPA clearance and permits for, was true high speed. It was to be electrically powered, on a fully-separated alignment, with a design top speed of 180 mph. It was deemed eligible for an FRA construction loan, but the Republican Congress ordered FRA to deny the loan. FRA couldn't do that, legally, but they could stop processing the paperwork, which is what they did. The project went into limbo because commercial financing of the amount needed was going to cost too much.

The Virgin Trains group that has taken over Brightline and now the former Desert Xpress project is downscoping it to 125 mph operation using diesel trains similar to Brightline's. Electric operation has been dropped. It's not clear how much downscoping of the construction work is going on, but a number of expensive features of the original line, needed to support high speed operation, will likely be modified, including grade separation through Barstow and the long summit tunnel at Mountain Pass. The original plans called for closely following I-15, within the median and adjacent in the freeway r/w in many places, to minimize impacts on adjacent federal lands; it's not clear how much of that will remain using diesel trains, which might not manage some of the grades as well as the electrics could. Bottom line: I'd expect a new NEPA document, or at least a comprehensive supplement to the previous one, to be needed to address the changes. Has Virgin included that in its plans for the project?

In the US, 125 mph is de facto high speed, or at least close enough for government funding. The only trains in the country that exceed 125 mph are Acelas in selected parts of the NEC. The only places in the country where trains can exceed 90 mph are the NEC, small parts of the Illinois corridor (Chicago-St Louis), and parts of the Michigan corridor (Chicago-Detroit).

Brightline at 125 mph (north end of the coastal FL corridor to Orlando) is a major addition to the "higher" speed mileage in the US. The starter desert line between Victorville and LV, even at only 125 mph, would almost double the mileage of non-NEC line that's operable at more than 90 mph. And frankly, that's what the US needs more of: higher-speed (but not high speed, with all the costs and political difficulties that drags in) regional trains in otherwise congested corridors. 125 mph is probably approaching the limit for diesels, so that probably is the practical limit for higher speeds in the US for the foreseeable future; adding catenary to shared lines, which most higher speed passenger lines are or will be at least in part, just isn't going to happen.