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I believe the main impact comes from Ubuntu using Linux 4.15 and GCC 7.3, while the other distros tested used Linux 4.17 and GCC 8.1+.

Different Linux kernel config options, probably. Different options can drastically affect performance.

Compiler flags mostly.

Also different compiler versions; I would expect gcc 7 vs 8.1 vs 8.2 to make a difference.