How can I make an existing persistant live USB installation load more of the OS to RAM?

Check out these instructions. To summarize:

align the partitions with the SD card erase block size (so the filesystems are aligned when created).
Move directories like /tmp and /var/log to RAM by making fstab entries. Also turn off access time tracking for / with the noatime option in fstab.
Link other directories like /var/cache/apt/archive and your browser cache to a place in /tmp.
Make RAM home based users for quickest access -- you may not explicitly write much in your home directory, but the GUI and browser certainly do.

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