hi @Mathieu
All calculations that iterate through issues are slow, as they always iterate through all issues.
You may want to check out another community thread with a similar issue (and formula):
There are limitations to how far you can optimize measures, therefore, yes, sometimes precalculation (JavaScript on eazybi side or, for instance, ScriptRunner on the Jira side) is the only way to get a result fast.
If you need our assistance in figuring out if you have room for the report/calculation optimization, please export the report definition and send it to support@eazybi.com
Best
Ilze / support@eazybi.com