DMS slow CDC apply

Have they tried to work on best practices? There are several very key best practices to improve the performance. https://docs.aws.amazon.com/dms/latest/userguide/CHAP_BestPractices.html

1. Migrating large binary objects (LOBs), Large object migration has a low performance. You can also look at row filtering https://docs.aws.amazon.com/dms/latest/userguide/CHAP_BestPractices.html#CHAP_BestPractices.LargeTables
2. Reducing the load on your source database and target, Did you check how is the metrics in the target database? if that is ingress slowly, perhaps this is related to a number of reasons. e.g. network connection from DB to on-prem, slow query etc. There is a network architecture below about on-prem for ref https://docs.aws.amazon.com/dms/latest/userguide/CHAP_BestPractices.html#CHAP_BestPractices.Rte53DNSResolver
3. It is always recommended to have multiple DMS job replicating across multiple table instead of having one DMS for whole database
4. You can also check out the batch apply mode, however, be careful to the limitation listed. Some AWS doc also mentioned with the batch optimized apply option, loading into the net changes table uses a direct path, which doesn't support XML type. As a workaround, you can use transactional apply mode. https://repost.aws/knowledge-center/dms-batch-apply-cdc-replication
5. Golden Gate is another third party option for oracle related migration https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/Appendix.OracleGoldenGate.html