The append is the only thing that is O(1), finding the deletion mask is linear (≠ is linear, isn't it?) and the actual deletion is also linear (⌿ is also linear).

       ]runtime -repeat=1s 'v←k1 ⋄ v⌿⍨←v≠2'

 \* Benchmarking "v←k1 ⋄ v⌿⍨←v≠2", repeat=1s
 ┌──────────┬──────────────┐
 │          │(ms)          │
 ├──────────┼──────────────┤
 │CPU (avg):│0.008491847826│
 ├──────────┼──────────────┤
 │Elapsed:  │0.008466372283│
 └──────────┴──────────────┘
       ]runtime -repeat=1s 'v←k2 ⋄ v⌿⍨←v≠2'

 \* Benchmarking "v←k2 ⋄ v⌿⍨←v≠2", repeat=1s
 ┌──────────┬────────────┐
 │          │(ms)        │
 ├──────────┼────────────┤
 │CPU (avg):│0.8333333333│
 ├──────────┼────────────┤
 │Elapsed:  │0.83        │
 └──────────┴────────────┘