add performance metric

Signed-off-by: haojinming <jinming.hao@pingcap.com>

content/docs/dev/concepts/explore-tikv-features/backup-restore.md

@@ -163,4 +163,26 @@ Please note that if data is stored in TiKV with [TTL](../ttl), and expiration ha
 
 TiKV-BR supports TLS if [TLS config](https://docs.pingcap.com/tidb/dev/enable-tls-between-components) in TiKV cluster is enabled.
 
-Please specify the client certification with config `--ca`, `--cert` and `--key`.
+Please specify the client certification with config `--ca`, `--cert` and `--key`.
+
+### Performance
+
+The backup and restoration are both distributed. So the backup and restoration performance can increase linearly with the TiKV nodes' count if the storage and network don't reach the limits. The following are some key metrics of TiKV-BR backup and restoration for reference.
+- TiKV node: `4C8G`
+- PD node: `4C8G`
+- TiKV-BR node: `4C8G`
+
+|Metric|v6.4 apiv1|v6.4 apiv2|
+|:-:|:-:|:-:|
+|Max storage size|50T|50T|
+|Backup speed|40MB/s per TiKV node|40MB/s per TiKV node|
+|Restore speed|70MB/s per TiKV node|70MB/s per TiKV node|
+|Performance impact|20% on QPS/Latency|20% on QPS/Latency|
+
+#### Performance tuning
+
+If you want to reduce the impact of backup tasks on the cluster, you can enable the [`auto-tune`](https://docs.pingcap.com/zh/tidb/stable/tikv-configuration-file#enable-auto-tune-%E4%BB%8E-v54-%E7%89%88%E6%9C%AC%E5%BC%80%E5%A7%8B%E5%BC%95%E5%85%A5) feature. With this feature enabled, BR performs backup tasks as fast as possible without excessively affecting the cluster.
+
+Alternatively, you can limit the backup speed by using the TiKV configuration item [`backup.num-threads`](https://docs.pingcap.com/zh/tidb/stable/tikv-configuration-file#num-threads-1) or using the parameter `--ratelimit`.
+
+See detail description in [`BR Auto-Tune`](https://docs.pingcap.com/tidb/dev/br-auto-tune).