A 512GB MicroSD is listed as an Amazon "Epic Daily Deal" for $80. You can get other brands for the same price, so I'm not sure if the extra speed would matter with MiSTer or one of the new ODE's? I usually just get the cheapest 
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It will take a while on xStation as well. There is a problem inherent to SD card file systems where too many items per folder level slow down scanning. How many items depends on how much cache there is, and for xStation, it's good for ~150 items.
I see so it is not a matter of speed but just an issuee with sd cards. oh well thanks for clarifying it!