The SD card is slower than the internal memory for a start and it's the number of individual books rather than the length of each that would cause any perceived slowdown... I have owned a few devices and I don't know one that wouldn't evidence some slow down with 6500 titles loaded ,especially on an sd card you will likely just have to live with it...... I doubt that any messing around with collections would change that and as already mentioned the un-rooted reader does not do hierarchical storage... the software treats your books collection as one big folder......
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