RE: What makes such a big difference?

One thing to do when you put back the logicals is check the keys. Put
the longest keys on first down to the shortest so that the AS/400 will
share any access paths.

For example, if you have a logical with

Cust#,ShipTo,Item#

And other

Cust#, Shipto

The As/400 will only create one access path.

This will only work if all the access paths are the same type. *MAX1TB
or *MAX1GB.

Also, IBM recommends that you create Access Paths as SQL indexes first
then create the logicals. SQL indexes are much more efficient.

When you are done, you can keep the SQL indexes or delete them. The
logicals will maintain the SQL indexes.

If you are doing this work anyway, might be something to look at.

I did this on a few files at a company I was consulting at. Ended up
saving like 100gb of storage just on the indexes because they were huge
and reduced number of indexes down by 10 or 20.

Surprising thing to me is that IBM does not have a command that does all
of the automatically.

You, also, indicated that you did not have the source. Programmers can
do unbelievably stupid things like reading the whole file in as update
to update a few records or reading files they don't need to. Without
seeing the source, pretty hard to know what people are doing.

I worked on a package where they had a multi-format logical over order
lines and two types of comments with a million comments. All the
programmers where reading the multiple format logical in update to
update a few hundred order lines so they were reading a million comments
they had no interest in and this was through the whole system. Geez.