[popcornmix]

#408 RPi 3, iso BD files still very slow (>1 minute typically) to start playing from an NFS sever (Synology) - other media players access and start playing same files in ~1 second.

ISO files will always be much slower than mkv. An mkv file has an index at the start with all the important data.
An ISO file doesn't and a lot more data needs to be parsed to work out what needs to be done.

Try setting "Blu-ray playback mode" to "Play main movie" in video/discs settings, and also enable "attempt to skip introduction before DVD menu" if you use DVD ISOs.