Jepsen is essentially a worst-case stress test for a consistent database: all the transactions conflict with each other. CockroachDB's optimistic concurrency control performs worse with this kind of workload than the pessimistic (lock-based) concurrency control seen in most non-distributed databases. But this high-contention scenario is not the norm for most databases. Most transactions don't conflict with each other and can proceed without waiting. Without contention, CockroachDB can serve thousands of reads or writes per second per node (and depending on access patterns, can scale linearly with the number of nodes).
And of course, we're continually working on performance (it's our main focus as we work towards 1.0), and things will get better on both the high- and low-contention scenarios. Just today we're about to land a major improvement for many high-contention tests, although it didn't speed up the jepsen tests as much as we had hoped.