We are in the process of upgrading from 8.8.1 to 8.10.4. As part of the migration we have also converted one of our large rule projects from Classic rule engine to Decision Service. We have deployed the decision service successfully. But the rule execution is taking 3 to 4 seconds on 8.10.4 as compared to less than 500ms in 8.8.1. Why is it slower in ODM 8.10 compared to ODM 8.8.

If for the first time execution it takes 3 seconds, the subsequent executions should take much less time than that, could be one tenth of that time. If all the subsequent execution take the same time, you need check the XU configuration for caching and asynchronousRulesetParsing.

Even the ODM 8.10 document states that this asynchronousRulesetParsing is set to true by default, but you need to check your system deployed ra.xml file for that configuration. It might have been changed along the way when you are using the file by other people.