I was driving a Tesla Model Y with Full Self-Driving (FSD) engaged in Standard mode on a wet roadway post-rain. Shortly before the incident, my vehicle had completed a lane change on FSD and was traveling with traffic flow. A vehicle ahead braked suddenly after reacting to another vehicle merging unsafely near a lane split. The stopping occurred quickly, reducing available reaction time. The FSD system did not appear to respond with sufficient braking force or early enough given the conditions. A forward collision warning sounded shortly before impact, but there was limited time to avoid the collision. The vehicle ultimately rear-ended the car ahead. I am submitting this report because of the systemâs delayed response in wet conditions, failure to ensure enough space on FSD- especially after a lane change and during a sudden/abrupt traffic slowdown may present a safety concern if experienced by other drivers.