Adding index on v6.6 is 5% slower than v6.5 when using tidb_ddl_enable_fast_reorg

[dbsid]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

add index on ossinsight dataset

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

no performance regression

3. What did you see instead (Required)

5% performance regression

4. What is your TiDB version? (Required)

v6.6

v6.6

[zimulala]

There was some jitter in between, but in multiple tests(doc), you can guess that this PR(#40711) caused a 2-3% performance drop. @you06 Please have a look.