The most important thing regarding speed is the priority set in your email activity. If this is set to high I definitely got faster email deliveries than in your test. The journey delivery duration was roughly the same as for triggered sends (or just some seconds slower). In tests I performed some time ago I got the following results (measured from the time the trigger is fired until the email notification from google mail arrived):
API-Triggered journey with E-Mail send activity (average of several test runs):
- Priority High: ~15s
- Priority Medium: ~2m
- Priority Low: ~6m 20s
Triggered send (average of several test runs):
- Priority High: ~11s
- Priority Medium: ~1m 30s
As Eliot mentioned, sending with high priority costs 4 super messages instead of one. Further information can be found in the Marketing Cloud Super Message Bundles document.
EDIT "Transactional Messaging API":
Recently Salesforce launched the Transactional Messaging API which is even faster than triggered sends with high priority. And another benefit is, that you don't have to waste so much super messages, as every send is only counted as a single supermessage. However, the downside is, that you cannot configure the messages via a user interface and have to do it via the API as well.
Further information: Transactional Messaging API