XF 1.5 Very slow import from vBulletin 4

I'm trying to import data from my vBulletin 4 installation, but the importer is very slow: in like 10/12 hours the user import is already at 71% and I have like 300.000 users, I'm worry for what I have to wait for import the topics and messages...

I don't think it's the "hosting" because we have a dedicated server with Intel Xeon CPU, SSD and 32GB RAM.

Any advice? Is this normal?