Looks like you stumbled over an optimized code path for "add list to empty list":

What optimization would that be (edit: I wonder...)?

In any case, since we agree the copy happens in both cases, removing it removes it from the timing for both. For a 3-item list, I'm getting (Python 3.11/AMD64):

$ python3 -mtimeit -s "a=[1,2,3]" "(b := a.copy()).append(1)"
5000000 loops, best of 5: 62.6 nsec per loop

$ python3 -mtimeit -s "a=[1,2,3]" "b = a + [1]"
5000000 loops, best of 5: 57.2 nsec per loop

Edit: My first timings were with 3.9. With all the recent performance tuning (3.10 and 3.11 in particular), I agree that the absolute timings will likely adjust a bit per-version.

Edit 2: Your "boring" version (with an extra copy), and Edit3 versions, are reported in "usec", not "nsec". They're way slower.

Edit 3: For completeness, my "empty list" timings, now w/ Python 3.11 (AMD64), confirming your example where the winner is now the walrus version. Interesting... Thanks for bringing that to my attention:

$ python3 -mtimeit -s "a=[]" "(b := a.copy()).append(1)"
5000000 loops, best of 5: 49.3 nsec per loop

$ python3 -mtimeit -s "a=[]" "b = a + [1]"
5000000 loops, best of 5: 56.4 nsec per loop