#1 — Best: Script Task using a!forEach()

This is the most efficient and recommended approach.

Executes entirely within a single expression evaluation
No process instances are created
No engine round-trips per record
Minimal memory footprint and near-zero process engine load

a!forEach() operates on the full dataset in memory, making it ideal when the complete list of records is already available. It effectively behaves like a functional loop and returns the transformed list in one operation.

👉 Why it wins:

Maximum performance, lowest overhead, and aligns with Appian best practices for in-memory data transformation.

#2 — Good: Start Process Smart Service with MNI (Parallel Execution)

This approach starts one asynchronous process per record using Multiple Node Instances (MNI).

Runs in parallel, improving overall execution time
Load is distributed across multiple execution engines
Each record is handled independently in its own process

👉 Why it’s strong:

Unlike subprocess, this method leverages Appian’s execution engine load balancing, preventing bottlenecks when handling large datasets.

👉 Trade-off:

Higher memory usage due to multiple process instances.

#3 — Moderate: Subprocess Node with MNI (Parallel but Single Engine)

Also uses MNI to process records in parallel, but with a key limitation:

All subprocess instances run on the same execution engine as the parent
Can lead to engine saturation under heavy load
Execution speed may degrade as volume increases

👉 Why it ranks lower:

Lacks cross-engine distribution, making it less scalable than the Start Process approach.

#4 — Worst: Synchronous Subprocess in a Loop (Sequential Execution)

This is the least efficient pattern.

Executes one record at a time (sequential)
Parent process waits for each subprocess to complete
Holds memory and process context for the entire duration

👉 Why it performs poorly:

No parallelism
High memory usage
Increased execution time
Single-engine processing

Official Documentation

Looping in Appian: https://docs.appian.com/suite/help/latest/looping.html
Subprocess Activity: https://docs.appian.com/suite/help/latest/Sub-Process_Activity.html
Start Process Smart Service: https://docs.appian.com/suite/help/latest/Start_Process_Smart_Service.html
Ways to Start a Process: https://docs.appian.com/suite/help/latest/Ways_to_Start_a_Process_From_a_Process.html