Does Postgres CHECKPOINT process affects streaming replication?
You are correct, the speed of promoting a standby is not affected by checkpoint timing on the primary. WAL is streamed to the standby as soon as possible, and writing WAL is mostly independent of checkpoints (the only connections are that a checkpoint writes a WAL record and removes obsolete WAL segments).
What can delay promotion is if there is a conflict that prevents replaying WAL on the standby, but that's something entirely different.