Reorg with empty table too slow

[Defined2014]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

create table t(a int);
alter table t add index i1(a), add index i2(a), add index i3(a);

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

Finish in 3s.

3. What did you see instead (Required)

Cost more than 8s.

4. What is your TiDB version? (Required)

Because of async commit, reorg will wait 2.5s to guarantee all txn started before ddl are committed. For multi-schema change, wait 2.5s is enough.