The high usage of Java Heap old generation lead to a full Garbage Collection every minute, hence the performance of the author instance is degraded
The issue comes from a memory leak in cached adapters in Sling.
This has been solved through https://issues.apache.org/jira/browse/SLING-7586 and included in AEM 6.5 and backported to 6.4.4 under the references GRANITE-23537 / NPR-27636
As an immediate solution, updating the 2 bundles org.apache.sling.models.api to v1.3.8 and or.apache.sling.models.impl to v1.4.10
These 2 bundles are part of a support fix (see above)