Process Model- split using AND gate

The process follows the logic, but the individual nodes are enqueued and executed by one of the multiple execution engines.

Is this just out of curiosity, or do you have the need to speed up things?

AFAIK, Appian does run one process instance on one execution engine - so even with AND parallel branches, all nodes are processed by that single engine, meaning:
True thread-level parallelism doesn't happen within one instance
Branches are queued and picked up by the same engine serially.
The parallel in AND gateway is logical, not true concurrent threading within a single instance.

True speedup only via background startprocess() - each gets its own engine instance.

Short answer: No, Appian does NOT force everything to run linearly.

Appian does support parallel execution with gateways, but it doesn’t guarantee simultaneous execution.

Appian process execution engine uses a limited thread pool, so parallel branches are scheduled based on available resources.

In your case, as the nodes are lightweight, both linear and parallel flows take similar time. You’ll see real performance improvement only when each branch has heavy operations like integrations or long-running tasks.