If you a large NSF database file, you should try to compact NSF database. IBM Lotus Notes program has a built-in Compact feature which you can use here to compress NSF database file size. And to split NSF database, you can try an easy manual trick. You need to use the Drag & Drop feature here to move mailbox content from one NSF database to multiple NSF database files. Refer to this article for some useful information - http://lotusnotesmigrationtips.com/split-nsf-database/

I hope the shared information would be helpful here.

Thanks and best regards.