When reading through some Juniper articles I discovered this on the page about LACP:
Note: On EX2300 and EX3400 switches, the LACP protocol must be configured with a periodic SLOW timer to prevent flaps during CPU intensive operations events such as routing engine switchover, interface flaps, and exhaustive data collection from the packet forwarding engine.
Something to think about when rolling out those switches. I rolled them out with lacp fast. Didn’t hear complaints yet 🙂