Upgraded SSD - Now It's Slower

A couple of days ago I cloned the pre-installed 128GB SSD onto a new, higher specification WD 500GB SSD. After cloning, I installed the new SSD and noticed it has markedly inferior 4K file speeds. The SSD is the boot drive plus programs/apps/games, while other data is stored on a regular spinning hard drive HDD. The only other detail is that Windows installed a security update shortly before cloning took place, but it’s hard to imagine that would have any significant effect on SSD speeds. So, why is the new, bigger/better/faster SSD running slower on 4K file performance??
The system:
ACER Aspire E15 Model E5-575G Laptop
Windows 10 Home 64-bit
Factory installed SSD - Lite-on CV1-8B128 (M.2 SATA III)
Newly-cloned and installed SSD - WD 3D Blue 500GB (M.2 SATA III)
Factory installed HDD - Seagate ST-1000LM024 1TB