Why did this just happen ?

USB uses the internal processor to handle everything. Speed and detail is all put on the processor.  This is like when you are moving music files onto an eternal HDD, on an older computer it will be so slow, put on a new computer, the speed is much faster. 

Then factor in all the new technology of a new computer, plus better CPU/RAM/HDD and updated software. 

USB is a horrible transport for just about anything, but it has become the standard. https://www.howtogeek.com/53587/usb-explained-all-the-different-types-and-what-theyre-used-for/

@waytoomuchstuff ....I've been interconnecting Winboxes to the audiophilia stack before spouse got a gig as the landscape maintenance manager for the complex we got to live in as part of her salary package....

It just 'happened' to be tied into a T3 cable to a local software design/implementation firm a mile away.....full motion videos f/t when the local cables would stststutt er with mpegs.....It was funny when they'd call to hype their service...

"S'cuse, but we're on a T3 link here...."  The smart ones would be impressed and wish us a good time...the others?

"Ask your super what a T3 is....and hope they keep you around..." and hang up.

Place was full of geeks, gamers, day-traders looking to trade seconds for dollar$....
Near zero latency....

The server had an music file directory that was Huge.....and got shut down during that whole issue that got some sued by the labels involved.
Those that saw it pending downloaded anything they wanted, but disc drives were still in megabytes and the eq of a t-byte drive would outcost the 'puter you were running....

'Ell, your cell is more capable now....and I'd already been doing 'room eq' for a couple of years prior to that stint....

@nyaudioguru That ifi iPurifier sounds like a worthwhile widget, but I'm concerned over the surge in esoteric 'puter cables that would undoubtedly price like audio cabling does now....

I'm backing up some of my more 'important' connections with opticals, just to see what may lurk there.....kinda leaves the turntables out of the picture, but analog be analog and should be enjoyed as such....

@mswale, yeah...USB ain't perfect, but 'market forces' can trigger all sorts of this'n that to occur...and if enough push for it, noisy power supplies can rise to 'phile levels...although having those rise in co$t will not be a surprise when they do....

My dedicated audio 'puter is already so non-SOTA, and I took it as far as I could manage at the time...and in the case that I've clung to since HOU/T3 time....(looks and sized like an average receiver/pre/amp and fits rack mounts....)

My primary guess would be that the newer Mac has a newer OS with improved drivers for the DAC, and that what you were using before was "crippled" relative to the driver. 

Either that or you find the effect of using a hub between your Mac and DAC to be euphonic and pleasing.  Just in general, USB hubs are not recommended for use with DACs, so you might want to connect directly.  

Hey @ BTW....if y'all are deep into your 'phile items, DIY'ing a 'puter isn't as deep a chore as one is already with your gear....more or less....

Pick your processor and match the m/b to it....select the suitable RAM chips, and stuff the slots FULL.
You want speed...there's charts online to guide that... ;)

Start 'fresh', you'll want your current lap or box for info and guides.

T-drives are ridiculous cheap lately.....but use a SSD drive for C:

There's audio cards available now that I couldn't afford or didn't exist at the time.

Find one...not just for 'now', but for Later,

What and to what to 'out' to is potentially another rung on the upgrade spiral....

Case in point:

Studios have been in digital for awhile now, with some analog diehards still in the mix.....

You Can't Beat Them...."Surrender!   ...but don't give yourself away..." ;)