The most common reason for a migration task running slowly is that there are inadequate resources allocated to the AWS DMS replication instance. Check your replication instance's use of CPU, memory, swap files, and IOPS .
Choosing optimum size of replication instance.
It covers the common scenario of a full load + CDC task.
Consider the following factors when choosing a replication instance class and available disk storage:
Table size – Large tables take longer to load and so transactions on those tables must be cached until the table is loaded.
Data manipulation language (DML) activity – A busy database generates more transactions. These transactions must be cached until the table is loaded.
Transaction size – Long-running transactions can generate many changes. For best performance, if AWS DMS applies changes in transactional mode, sufficient memory must be available to stream all changes in the transaction.
Total size of the migration – Large migrations take longer and they generate a proportionally large number of log files.
Number of tasks – The more tasks, the more caching is likely to be required, and the more log files are generated.
Large objects – Tables with LOBs take longer to load.