The biggie is the GPU. T420 has no oomph to run an external 4K display (at more than 24 Hz, i.e. in usable mode). Broadwell and newer do have the capability to run 2 of them.
The CPUs got less power-hungry. You can do the same work with less juice, so your battery lasts longer.
The SSDs with the new interfaces got much faster. There is simply no comparison between M.2 nvme drive and SATA3 SSD.
Sadly, wifi took a step back, with almost universal unavailability of anything better than 2x2 MIMO 802.11ac. In the past, MIMO 3x3 used to be available (Broadcom, but the option was there).
So yes, modern laptop is significantly different experience than few years old one, despite the CPU having the same GHz.
My point is that they're not noticeably faster for the things most people use their computers for. They still run browsers, Spotify, email, word processors and all of that just fine and last "long enough" on a charge.
The improvements have been incredibly marginal for most people.
Also, they do not have a problem with Electron apps. for most people, they are just apps like anythig else, and do not perceive any performance problems with them.
And where did Electron apps come into the discussion?
Even the Apple 2015 M.2 drives, which are "only" AHCI, but with much deeper queue (i.e. nonstandard), can do 1.something GBps and the difference compared to standard SATA SSD is noticable by normal users.
Electron apps are something normal people use and some of them compain about the speed/lag/memory usage. Usually those with older computers, who think nothing changed in the last years ;).
If you do the first, you will do also the second. It makes a difference even when launching Excel or Firefox.
Between SATA SSD and PCIe? Sure, for extremely IO-heavy tasks there's a difference to be felt, but not really in everyday use.
Sure, not the very newest hardware, but it's not like laptops have gotten significantly more powerful for anything that actually matters, for the last couple of years.