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I have saved my original media on an external ssd and created optimized media in the same place. Now I finished editing on my old mbp 2012 and would like to test playback performance and video quality differences of the original files on my old MacBook and a new MacBook Air m1. Is there any option to use the original files instead of optimized media without deleting optimized media? I tried to rename the folder but FCPX notices and relinks automatically.

You could move them to the trash temporarily, just don't empty it until your test is done.

Thanks! This worked out, even a new MacBook Air m1 does not playback Sonys XAVC-S UHD 10bit 50p footage well when its not optimized. There's just a little color correction and a broadcast safe filter applied on the timeline - the project is 50i HD. I did expect the MacBook to handle h264 based codecs better - but I must admit it only has 8gb ram, maybe that's the cause. When I use ProRes optimized footage from the same SSD it plays back well, so SSD speed is not the problem.