Very slow query on v18 Server after switching to Catalina

A complex query which ran fast enough (OK, a few seconds) on v18 Server on a Mac Mini with OS High Sierra is taking 30 seconds or more after the mini was upgraded to Catalina.

The same query has also been running happily for many years on previous versions of 4D and Mac OSs (most recently it ran on v16 R5, v17 was skipped).

Since v18 was was OK on High Sierra I am inclined to look towards Catalina before I spend time on the reviewing the query methods.

Has anyone experienced anything similar with Cataline and Server v18?