Fast and Slow operations are separated by the threshold configured either for the analyzer as a whole (in the AMD settings), for the software service as a whole (in the software service options), per URL (in the case of HTTP/S protocols; in URL monitoring settings), or per parameter combination (if these are used in URL Monitoring to distinguish between URLs).
By default, for HTTP, a page load (operation) is considered slow if it loads in more than 8 seconds.
What protocol do your transactions use?
I think for Enterprise Synthetic, the thresholds are defined on the ES side, not the DC RUM side. DC RUM is just picking them up from the transaction definition.
Try charting the Max slow transaction threshold metric. By definition this is the maximum of a slow transaction threshold. If the transaction time is longer than the threshold, the transaction is considered to be slow. Does this correlate with the ES configuration?
The CAS will report a transaction as:
1. "Fast" if a transaction response time is below the performance threshold (50 seconds, in this example)
2. "Slow" if a transaction response time is >= the performance threshold and < the availability threshold (55 seconds, for example)
3. Unavailable (giveup) if a transaction response time >= the availability threshold.