It's mostly the SSD. Linux is great but the disk is a real choke point for a lot of system performance.
On Windows 10 it's definitely worse for sure. I don't know what they did since Win7 but it's pretty much unusable without an SSD now whereas a Debian would still run quite okay although slower.
* Modern windows has a really hard time on spinning rust, they'd have seen extreme performance improvements going to windows on an SSD
* Linux deals better with spinning rust, they'd have seen performance improvements going to linux on spinning rust
The SSD is still going to be most of the performance improvements. The gap between a 5400 HDD and an SSD (even more so NVMe) is just ridiculous on every single metric, there's orders of magnitude differences in throughput (especially on random reads or writes) and even more so latency and concurrency.
For sequential transfers, yes. But that's hardly ever the relevant figure for whether your system feels fast. You should be looking at the random read performance, where your hard drive is good for ~120 IOs per second and your SSD is good for several thousand at a minimum.
The problem is that since the majority of machines have SSDs now developers no longer optimise the number of IOPS or care about it; or notice it at all - as SSDs have orders of magnitude more IOPS performance than a HDD. Combine that with many people having quite a few background programs that will spew IOPS randomly you'd be surprised how fast the disk gets slow.
Even MacOS which until recently (and maybe even now) sells machines with a HDD as standard; with pretty much no extra software installed is unbearably unusable on a HDD.
SSDs are basically required at this point.