just upgraded iMac late 2015 ram from 12 GB to 28 GB and it became too slow to use
You should have matching DIMMS in order to maximize performance / dual channel functionality.  Also note that RAM is going to operate at the lowest speed of all the DIMMS, which is another reason why they should all match.  

That being said, it shouldn't be operating that slowly.  I would start with removing the original 4 GB stick, possibly doing a PRAM reset, and maybe shuffling around which slots they're installed in, since the problem only starting showing when you used the two vacant slots with the new RAM, if I'm reading correctly. Also confirm that the system is recognizing all the memory you installed.

Finally, it's possible you got bad/defective memory.  You can also try running diagnostics to rule out logic board issues etc.