I "inherited" a Dell XPS 15 laptop from a previous client, and decided to upgrade the 32 SSD m.2 cache drive to a larger and faster one. Noticed also that the battery seemed to not be keeping a charge, so ordered a replacement battery as well.
When I opened the laptop up, I noticed that the battery I received (generic, eBay) was twice the size of my existing battery inside the computer.
Fortunately, my plan was to just use the existing 1TB drive in there as slow, secondary storage for music, videos,, photos, etc because what I had to do (to make the battery fit), was remove the TB HDD to make space for the larger battery. Worked out well really, because while I gained a slight bit of weight, I gained a LOT of extra battery power, since those HDD's do suck down the juice.
Added a modestly sized 256GB SSD (m.2 NVME) and wow...completely different machine. The speeds are ridiculous now, even though they were not bad before.
I would like to be able to say that upgrading from a last gen SSD (6GBps) to a lower tier NVME SSD drive (16GBps) made a noticeable difference, but the way Dell configured the cache SSD in RAID with the HDD, makes that hard for me to say. Most online articles I have read say that there is not nearly the noticeable jump in speed from old SSD to NVME, as there is from HDD to (old) SSD.
I do notice that there is already a misconception out there about SSD formats and speed. m.2 is just an interface that can include older gen SSD drives that are no faster than their non m.2 counterparts (like the cache drive in my XPS that I replaced). NVME is the new SSD interface that has theoretical speeds that are more than double the speed of older SSD formats.
Bottom line, just because a drive has an m.2 connector, does not mean it's faster than your 5 year old Sandisk SSD. NVME definitely does mean that, however.