Dell EMC DataDomains do have good ingest performance, you can typically throw hundreds of streams at them and they'll greedily gulp it down.
And it is true that they are dog slow at restoring data. The reason? They are deduplicating appliances, you have to rehydrate the data, and this can take a very long time depending on the number of blocks needed that constitute the backup. Some blocks may be shared between hundreds if not thousands of backups. Dell isn't one to talk about their downsides much.
They are best used not as _primary_ storage, but secondary (think 3-2-1 backup rule). You should have some fast nearline storage available for recent backups that require really low RTO (recovery time objective).
Depending on the application used for backup, it may not have had native Postgresql WAL processing. This and may have had to be image based which slows down the process and requires some additional scripting.