These old 2010's systems support.SATA 3 Gb/s whereas the WD Blue SSD is highly likely to be of SATA 6 Gb/s which maybe causing a bottleneck. You may need to find an SSD that can auto switch to 3 Gb/s speeds (Samsung is a good brand that can do this).
https://everymac.com/mac-ssd-storage-upgrade-guide/macbook-pro-hard-drive-ssd-storage-upgrade-info.html
This maybe related to your issue.