Comments - Phenomenon of read-only queries performance
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The performance gain in the first scenario on switch from R/W to R/O could be due to usual database "warm-up".
When compared to standalone server, galera node (assuming that the test was run against a galera cluster with a single node) should perform as good as standalone if not worse. So, I suspect the two servers were run with different configurations. One option would be to compare servers' system and status variables using SHOW VARIABLES & SHOW STATUS respectively.