I haven’t used the relinking optimized media feature a bunch but as a workflow thing: If you’re making A LOT of optimized media that you want to move around it would probably be better to make proxies and have a proxy workflow instead.
Well if you change the custom preset to something else and then want to use the H264 master preset at some point? It’s just a preset that you can either use or choose not to use. Personally I wouldn’t export anything H264 from Resolve to actual delivery. The quality ain’t so hot.
When you select a preset you can see the settings it uses in the video and audio settings. Apparently it defaults to Quicktime H264 with a best quality setting instead of a constrained bit rate like in the Vimeo and Youtube presets.
If you have compressed footage your computer is working hard to decode it. Especially if it’s 4K. DNxHD, Cineform and Prores take more space but are easier to decode and are visually lossless.