I know you can disable some checks to make slapadd faster. But I think in my test vm with limited disk iops, it looks like this disk io is the problem. I am not sure how slapadd adds entries, I guess one at a time? You could get a significant improvement by reading more entries and writing more entries at once? And if such a batch transaction fails you can always go back to submitting the transactions of the batch one by one, to identify which one is failing.