create table t(a Int64, b Int64, p DateTime) Engine=MergeTree PARTITION BY toYYYYMM(p) order by p;
create table t2(a Int64, b Int64, p DateTime) Engine=MergeTree order by a;

insert into t(a, p) select number, toDateTime('2021-01-01 00:00:00') + number/100 from numbers(100000000);
insert into t2(a,p) select number, toDateTime('2021-01-01 00:00:00') + number/100 from numbers(100000000);

21.4.1.6248
select a,b from t WHERE p >   (select max(p) from t2 )  limit 200;
Elapsed: 0.192 sec. Processed 100.00 million rows

21.2.2.8
Elapsed: 0.201 sec. Processed 100.00 million rows

20.8
0 rows in set. Elapsed: 0.058 sec.

21.4.1.6248

select a,b from t　WHERE p > toDateTime ( (select max(p) from t2 ) )　limit 200;
Elapsed: 0.024 sec.