progress update is too slow for tables that are already available.

[hehechen]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

1. Create 50k tables with TiFlash replica.
2. Create table A with TiFlash replica.
3. Import data into table A with lightning.

2. What did you expect to see? (Required)

The progress of table A in information_schema.tiflash_replica will change in 5 minutes.

3. What did you see instead (Required)

The updating of progress for table A in information_schema.tiflash_replica is too slow, progress didn't change in 30 minutes.

4. What is your TiDB version? (Required)

ec93697