mmy late 2015 imac retina 4k is running excrutiatingly slow

I have a late 2015 iMac retina 4k which has never been quite as fast as i expected it would/should be. Has The diagnostics report was fine . Has 3.3 GHz Intel core 7, 16 GB Ram, 500 GB SSD. Not running any complex programs or graphics heavy demand. Used BlackMagic speed test and all looks good. No major startup demands. Any insights?

iMac, macOS Sierra (10.12.4)
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