Slow thumbnail / preview generating


i have few months old DS2422+ with 23 TB of data. Synology (upgraded with 10gbit card) is connected to mac via Lan. I am accessing files via SMB3.

My problem is that thumbnail generation takes forever.
Example: On my mac in Finder i open folder with 500 photos in .raw format ( 25mb per image ). Few images get preview instantly, but for most images takes 5 or more seconds to show thumbnail.
To generate all thumbnails in takes 20 min. In resource monitor i cant find any resource overload.

I set thumbnail generation in DSM on medium quality. But there is no difference.

Do anyone have any idea what is wrong and how i can speed up thumbnail generation?

Thanks for help.