A commitment protocol orchestrates the execution of a distributed transaction, allowing each participant to "vote" on the transaction and then applying a pre-specified rule to decide the outcome (commit or abort). A nonblocking is able to correctly terminate a transaction at all operational participants in the presence of any number of benign processor failures. Herein, we derive strong lower bounds for both non-blocking protocols and their less fault-tolerant blocking counterparts. Results on message complexity are both surprising and encouraging: the message complexities of the two classes of protocols are identical. Results on time complexity were less encouraging: nonblocking protocols are approximately 50% more expensive. However, we show how to overlap non-blocking executions of interfering transactions and thereby reduce their extra cost.