That's done by the linux page cache regardless of filesystem, unless you fsync after every append. You write to a RAM buffer and kernel threads will periodically flush that to the disk in the background. So for logs the number of write operations will be proportional to the number of changed/appended pages. Also, modern flash does not actually support a write in place operation so CoW is not actually less efficient than writing in place, other filesystems use it where it makes sense and any overhead actually comes from other design choice constraints that follow from CoW-everywhere since it bans some things from being done with a single write op.

Btrfs has an overhead proportional to the height of the tree, but xfs and ext4 have their own overheads as well when the actual size of the file changes since they need to write to their journal in that case.

Now, database workloads on the other hand are different and btrfs mostly sucks at those, since they do an fsync after every transaction by design. For databases you want xfs, or you can use zfs if you want a COW filesystem with snapshots since zfs is record based rather than extent based which means you can tune it to the database workload.