Update/delete operation throughput can severely (>10x) decrease when the change stream pre-images are enabled and more than one collection with change stream pre-images enabled is/has been active in terms of updates/deletes performed. 
The problem manifests almost always in throughput/latency change stream with pre-images tests. The impact can be seen on the second and subsequent test runs within a task. There is some randomness aspect caused by random assignment of unique ids to collections. The test sample in which the problem manifests: 15_whenAvailablefdbc_preImage_1c_avg_latency.