The large catalog
Do you keep all your photos in only a single catalog? I recently started spinning off my photos into separate catalogues (by year) to help with just seeing less, myself.
I'm also hyper aggressive about deleting rejected photos every few months; I think I'm only at 20,000-30,000 photos these days, when I used to be close to 100k, simply because what's the point of saving rejected photos I'm never going to look at again?
The bigger red flag for me is your hardware. I'd consider upgrading that first. USB HDD's are not the most reliable, and storing media you're actively using on HDD's is going to be slow, especially over HDD. If the USB drives are SSD drives that will help, are they running over USB-C, USB-A, Firewire?
I keep my active library on an SSD, and backup to a home server running HDD's. But older computers with lower RAM pools are going to struggle to keep up with modern memory hungry programs. I believe C1 (amongst others) can also offload graphical processing to preview to GPU's these days, and if you're running an older machine like you said, that's not going to be an option. Previews on my version of C1 23 render in about as fast as I can click through (even the mega 61mp files taken with my A7R V), but I also have a processor that's only 2-3 years old and a high end AMD GPU to help (with a crapton of GDDR RAM).
The yearly catalogues help because it's less that your computer will need to load into active memory and manage. This will help a little with flipping through things.
I'm only talking about speed with regard to SSD's. I'm not worried about reliability. Your typical 7200RPM HDD is going to have a data transfer rate at around 80-160MB/s. USB 3.0 has a theoretical maximum of 600MB/s, USB 2.0 has a theoretical maximum of around 30MB/s, if I recall correctly. Typical SSD's vary in transfer rate, but I know mine for my C1 library is 580MB/s (though connected over SATA theoretical Max is 6GB/s, that's not realistic to hit on my PCIe version).
So if you're talking an SSD over USB 3.0 or in your computer, you'll see incredibly different transfer speeds; if you're working over 3.0 or higher (which you said you were) you should see a huge step up in individual file accessibility with SSD's; that being said, previewing is going to be based on cache and hardware render, which on an older Mac may be the bottleneck you're hitting here. C1 has different overhead and uses hardware differently from FRW I think, and C1 has a lot of other tasks it's hitting your computer with besides just rendering the previews.
Like I said, it takes less than half a second for a 60 megapixel image to render on my machine. But we're operating very different hardware too (i7-9700K & a RX 7900 XTX with 24GB GDDR6), and I know that's playing a role.
I suspect you're being bottlenecked by the older processor & rendering solution in your Mac, but getting onto using smaller catalogues with an SSD should help you get some improvement.