The main factor would probably be that n8n has to wait until it receives a response from the server to which you make that request via the HTTP Request node. If you just want to see the speed of n8n itself, it is best to deactivate that HTTP Request Node but still leave the Webhook-Node set to return data of the last node (which would be in this case the Webhook-Node itself).
The only other thing I can currently think of that could cause a slowdown would be Postgres and the connection to it. If you also want to rule that out it is best to test with an SQLite database.