WebAccessibilityObjectWrapper accessibilityAttributeValue:AXPosition {
objectID 113
(role ScrollArea)
(parentObject 0)
(identifierAttribute )
(address 0x4b85da800)
(wrapper 0x15c11eec0)
} AXIsolatedTree::create exceeded ExecutionTimeThreshold 19.61s
} AXObjectCache::buildIsolatedTree exceeded ExecutionTimeThreshold 19.61s
} WebAccessibilityObjectWrapper accessibilityAttributeValue:AXPosition exceeded ExecutionTimeThreshold 7.36s

Note that the AXPosition request comes before the creation of the full tree completes, and the call goes to the new isolated object for the ScrollView which does not have this property cached, and hece has to hit the main thread.