I'm not familiar with 'Progress' but I suspect that it's not optimizing your code before it runs it. By changing the location of the table in the list in your from clause, I suspect you're changing the execution plan of the SQL query. Alterations to the execution plan can make huge differences to time to execute. (It's essentially the basis of query optimization)
This is one of the reasons that I no longer use the where clause join syntax. I used to have this problem regularly in Oracle. I find I have better success with optimization, and understanding the execution plans when I write explicit join clauses in the from clause, and not in the where clause. (It's also quite a bit easier to read and understand)
I agree with Ryan. I have no knowledge about Progress, but I know that some db:s are very sensitive to the query matching the indexes available. If it can't find an exact matching index the db will perform a table scan instead, which could be very time consuming.
I recall working with AS/400 years ago, and if you for example had an index on a table covering field A, B, C, but only used A and B in your query the index wasn't used. By including the field C in the query (even though not making any filtering on the field) so the query exactly matched the index you could see that the index was used, hence a huge performance boost.
It could also be that you need to rebuild your indexes, or other issues related to the upgrade. I suggest you turn the question to your Progress consultant which can help you out tuning the db for you, if you're not lucky finding a Progress expert here on the forum 🙂
