Do you only have the one drive? Which ever drive you are using for Scratch space, go to its root directory and look for old Photoshop temp files. These can get left behind when Photoshop does not close properly, and with just 16Gb RAM, you are going to be using a LOT of scratch space. Best rule of thumb is to close Photoshop, and any Photoshop temp files still there can safely be deleted.
As Kevin says, Smart Objects can slow things down. It keeps coming down to having what today we'd consider limited RAM as it means more use of Scratch space. If you are using just the one drive, then Photoshop (Photoshop does not have all modules loaded to RAM when opened) and Windows are reading and writing to that drive at the same time as Photoshop reads and writes to the temp files. We used to think that SSDs were fast compared to mechanical HDDs, but modern NVMe drives are orders of magnitue faster. I use a pair of Samsung 2Tb 980Pro drives for boot and Scratch space, and they have up 7000Mbs performance! It's not that long ago since HDDs might only have 80mbs and a SATA SSD might run a hundred times slower than a modern NVMe drive. That makes a huge difference when you have limited RAM and work on big projects.
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