It's probable the old Windows 7 install was 32-bit while your fresh install of 10 would have defaulted to 64-bit. That combined with 10's naturally higher memory requirements means the system has less overhead to work with.
It doesn't and never will. I've used them side by side for a few years and went back to W7 for productivity.
Interestingly enough, Lubuntu LXQt feels snappier than either system.
I'm not sure if this is because Windows memory usage is a lot more efficient now, or if the newer processors' performances can cancel out the RAM capacity bottleneck, or if PC4-25600 + NVMe pagefiles are simply fast enough, or if manufacturers are spreading thinly during the chip shortage. but it's certainly an ongoing trend
Mother I law bought a machine with 4GB of ram, which was fine before windows 10. Now it spends all day doing page/sysfile swap from its mechanical hard drive. Basically unusable.
So here in my pocket is an 8GB stick of DDR3 sodimm for later.
And 4GB is enough for a blank windows 10 install doing some OS things and browsing. I don't think more memory helps that scenario.
[0] https://en.wikipedia.org/wiki/Physical_Address_Extension#Mic...
I mean, everyone uses a browser, even if they use nothing else, and browsers gobble up RAM like crazy.