How to list files recursively and sort them by modification time?

Use find's -printf and sort on a reasonable date format:

find -type f -printf '%T+\t%p\n' | sort -n

This should minimize process forks and thus be the fastest.

Examples if you don't like the fractional second part (which is often not implemented in the file system anyway):

find -type f -printf '%T+\t%p\n' | sed 's/\.[[:digit:]]\{10\}//' | sort -n
find -type f -printf '%T+\t%p\n' | cut --complement -c 20-30 | sort -n

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EDIT: Standard find on Mac does not have -printf. But it is not difficult to install GNU find on Mac (also see that link for more caveats concerning Mac/Linux compatibility and xargs).

Here is a method using stat as @johnshen64 suggested

find . -type f -exec stat -f "%m%t%Sm %N" '{}' \; | sort -rn | head -20 | cut -f2-