How to improve cockroach performance with pgbench?

Hello,

I've been playing with CockroachDB, a distributed database system which is
more or less compatible with Postgres because it implements the same
network protocol. Because if this compatibility, I have used pgbench to
setup and run some tests on various AWS VMs (5 identical VMs, going up to
a total 80 vcpu in the system).

The general behavior and ease of use is great. Data are shared between
nodes, adding a new node makes the system automatically replicate and
balance the data, wow. Also, the provided web interface is quite nice and
gives hints about what is happening. They implement an automatic retry
feature so that when a transaction fails it is retried without the client
needed to know about it.

All this is impressive, but performance wise I ran in a few issues and/or
questions:

- Loading data with a COPY (pgbench -i) is pretty slow, typically 3
seconds per scale whereas on a basic postgres I would get 0.3 seconds
per scale. Should I expect better performance, or is this the expected
performance that can be achieved because of the automatic (automagic)
replication performed by cockroach? Would it be better if I generated
data from several connections (hmmm, pgbench does not know how to do
that, but the tool could be improved if it is worth it)?

- I'm at a loss at finding the right number of client connections to
"maximise" tps under reasonable latency. Some of my tests suggest that
maybe 4 clients per core is the best option. For a standard postgres,
a typical client count would be larger, typically around 8-10 per
core.
Is this choice reasonable for cockroach?

- The overall performance is a little bit disappointing. Ok, this is
a distributed system which does automatic partitioning and replication
on serializable transactions, so obviously this quality of service must
cost something, but I'm typically running around 10 tps per core (with
pgbench default transaction), so a pretty high latency, and even if
it scales somehow, it which seems quite low.
What I am doing wrong? What should I check?

- Another strange thing is that the steady state at full speed is quite
unstable: looking at instantaneous performance, the tps varies a lot,
eg between 0 and 4500 tps, more or less uniformly, i.e. the standard
deviation is large, say 1000 tps stddev for a 2000 tps average
performance.

Basically, any advice about cockroach configuration and running pgbench
against it is welcome!

Thanks in advance,

--
Fabien.