There's an obvious tradeoff. With a 4K page size, the breakeven point is around 768 bytes per record. For larger records, the WAL approach is less efficient than LMDB. For smaller records, the WAL is more efficient in terms of write amplification. All thoroughly explored here http://www.lmdb.tech/bench/ondisk/

It is: two writes for write ahead log vs. log-n (tree-height) writes for CoW

In a well designed database system (hardware!) you aren't going to use the same CPU, disk or controller to write the log and the data structure. So the performance penalty if any is going to be low.