Well, when the number of records increases then not only the join construct will take more time, but any other construct as well, and I bet relatively more than the join.
If proper index access is used throughout, then runtime should only increase slowly and linear, but not exponentially.
I'm not getting paid by joins to promote them, they have served me (and my clients) well for many years, so I'm trying to share the benefit, and also speak up against the simplistic performance rules that keep circulating.
select * from BSEG into corresponding fields of table lt_bseg. Performance will be disastrous.
As long as your WHERE-condition uses BSEG primary key and you declare lt_bseg containing only the four required fields, then performance will be OK, this is what I was trying to show. This is irrespective of the order of the fields in the database and your internal table, as per my observation.
Thomas