Lock convoy in order-pricing service

A .NET order-pricing service that's been stable for a year suddenly develops a p99 latency cliff during the morning rush: p50 stays at 8ms but p99 jumps from 40ms to 900ms once concurrent requests cross ~200. CPU is only at 35% across all cores, GC is quiet, and downstream calls are fast. The dashboard shows a high context-switch rate (180k/s, up from 20k) and a large number of threads in the 'wait' state on a single lock around an in-memory price-table cache that every request reads under the lock. The only recent change was a config bump raising the thread-pool min from 50 to 400 to 'handle more load.' How do you triage and mitigate?