So they are sent in bulk and not in parallel. Todo: Double check this is not already the case. Mostly interesting for content impressions, not interactions.
We need to do this in Matomo 4 since otherwise replaying tracking requests may break for some users as we'd do more POST requests compared to GET requests which can be replayed. In Matomo 4 users can then disable queue if they want to (they could already now but we'd not change such an important thing in a minor release)
Noticed we are already sending content impressions in bulk so there is actually no issue in doing this in 3.X already especially since https://github.com/matomo-org/matomo/pull/15860 doesn't work.