According to aja s data rate calculator 8 bit uncompressed is about 28 mb second.
Aja control room dropped frames.
I was able to get 8k output to display from premiere pro but it dropped frames during prores playback even at 24p.
Aja control room is unified cross platform software for ingest playback and output with aja products.
You should not be experiencing dropped frames.
Aja control room is designed to be used with aja hardware devices and it can also operate as a stand alone media player with full screen playback and host system audio monitoring supporting high quality professional media file types and formats.
Using aja s control room software i was able to get smooth 8k playback of prores hdr files at both 24p and 60p which is quite an impressive accomplishment on my older generation workstation.
Double check under the fcp systems settings to be sure that the xserve is set as your scratch disk.
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You xserve should be able to sustain over 120 mb second.