The problem occurred after a recent upgrade of Clarity (15.3 to 15.8) and the database (Oracle 12c to 19c).
Release : 15.8
Component :
Compare execution plan of prior and current version.
The execution plan for 12c was much lower than that of 19c, therefore it was suggested to set the compatibility mode to 12c.
Afterwards, the timesheet actions were much improved.
https://techdocs.broadcom.com/us/en/ca-enterprise-software/business-management/clarity-project-and-portfolio-management-ppm-on-premise/15-9-1/installing-and-upgrading/end-here-common-steps-after-all-new-installs-and-upgrades.html