Why does asset import take so long?

Any ideas why import of 1Kx1K png could take 10 minutes (584 sec)? See the log:

 Updating Assets/__TEMP__/AssetBundleRGB_/J10_sub2x2.png - GUID: 3a0483eef00a2411ca5be8ab1ef18c4c...
 WARNING: ETC texture format is not supported, decompressing texture
 ----- Compute hash for Assets/__TEMP__/AssetBundleRGB_/J10_sub2x2.png.
  done. [Time: 535950.312500 ms] 
 Unloading 123 Unused Serialized files (Serialized files now loaded: 0)
 System memory in use before: 224.1 MB.
 System memory in use after: 224.4 MB.
 
 Unloading 125 unused Assets to reduce memory usage. Loaded Objects now: 5112.
 Total: 8.159921 ms (FindLiveObjects: 0.713217 ms CreateObjectMapping: 0.122615 ms MarkObjects: 7.264199 ms  DeleteObjects: 0.058721 ms)
 
 Refreshing native plugins compatible for Editor in 48081.09 ms, found 3 plugins.
 Preloading 1 native plugins for Editor in 1.42 ms.
 
 ----- Total AssetImport time: 584.332947s, AssetImport time: 536.141296s, Asset hashing: 0.003174s [1.5 MB, 470.160217 mb/s]

This is part of an automated process which copies png files into Assets folder, reimports the asset, creates asset bundle and exports it. It worked ok with Unity4. It is crazy slow with Unity5. Any ideas?