What are the key differences between ACH payments and CHIPS transfers?

CHIPS is basically RTGS (Real-Time Gross Settlement) System for large value, low-volume, instant clearing, whilst ACH is low-value high-volume batch clearing payments. Both are US concentric.

As per Wikipedia:

The Clearing House Interbank Payments System (CHIPS) is a United States private clearing house for large-value transactions. By 2015 it was settling well over US$1.5 trillion a day in around 250,000 interbank payments in cross border and domestic transactions. Together with the Fedwire Funds Service (which is operated by the Federal Reserve Banks), CHIPS forms the primary U.S. network for large-value domestic and international USD payments where it has a market share of around 96%. CHIPS transfers are governed by Article 4A of Uniform Commercial Code.

Source: Clearing House Interbank Payments System | Wikiwand

ACH, on the other hand, is batched, a payment system that can do low value, but in high volume (because the fees is so low).

Again, as per Wikipedia:

An automated clearing house (ACH), or automated clearinghouse, is an electronic network for financial transactions, generally domestic low value payments. An ACH is a computer-based clearing house and settlement facility established to process the exchange of electronic transactions between participating financial institutions. It is a form of clearing house that is specifically for payments and may support both credit transfers and direct debits.

ACHs are designed for high-volume, low-value payments, and charges fees low enough to encourage the transfer of low-value payments. The system is designed to accept payment batches, so that large numbers of payments can be made at once.

Source: Automated clearing house | Wikiwand