Just after upgrade from 3.3.14 to 4.0.12 I see 20% more CPU usage and request processing time on production.
Can be some non-optimal settings. But with simple framework upgrade I got a performance penalty
(150k unique visitors per day, 6Gb of memory).
In same time application use 30% less memory.
UPDATE:
performance issue caused by broken hibernate second level cache for classes with inheritance
https://github.com/grails/grails-data-mapping/issues/1594
UPDATE 1:
After fix of cache issue I see CPU usage improvement. Grails4 use around 5-10% less CPU compare to grails 3.3.14
I think, upgrade to grails 4 recommended