Hello, I migrate from 5.5 to 9.
I have a diary, with 1 relation for every half hour from 8:00 AM to 8:30 PM: 26 slots x day and format for "weekly" and "daily workgroup" visualization. Every slot is a relation: 26 [daily] + (26*7)[group 7 users, daily]. In 5.5, multifile solution, with older hardware, all was reasonable fast. The solution, automatically converted, is similar in speed. The solution, redrawn from skratch in 9 is painful slow. In the old version, the key is a calculated text field that combines the name of the user, the day, and the hour.
In the new version, I have catch the ability to set relations with more than a key: 1 = user, 1 = date, 1 ≥ start hour, 1 < end hour, without calculated field. And all is in a monofile/multitable solution.
I think the point is bidirectional relations: every single change in the the record (modify data, or user, or hour) probably start a recalculation for all the relations... every when this is unnecessary; or may be that a multikey relation is so slower than a single key relation?