For debugging purposes, this is what I have:
When trying to run this smart rule (with 2 items matched) e.g. via Applescript using
tell application id "DNtp"
perform smart rule name "Resources: process inbox"
end tell
… it will beachball immediately, hold the whole app and eventually time out. It also seems very similar to this issue @Lynx was having a while back: Error "on performSmartRule (Error: Error: AppleEvent timed out.)" when running a smart rule wit a javascript script every minute - #13 by mdbraber