use tempdb
go

set nocount on;
set xact_abort on;
go

if OBJECT_ID('tempdb..tmp_zs_456') is not null drop table tmp_zs_456
go
create table tmp_zs_456 (id int primary key identity(1,1), szam int default 0)
go


declare 
	@id int = 0, 
	@from_id int, 
	@to_id int = 10000, 
	@end_time datetime, 
	@start_time datetime

while @id < 1000000 begin
	set @id += 1
	if @@TRANCOUNT = 0 begin tran
		insert into tmp_zs_456 default values
	if @id % 5000 = 0 AND @@TRANCOUNT <> 0 commit
end

if @@TRANCOUNT <> 0 commit

-- Loop 1: the execution time is ok.
select @start_time = GETDATE(), @from_id = 0
while @from_id < @to_id begin
	select top 1 @from_id += 1 from tmp_zs_456 
	--select top 1 @from_id += 1 from sys.databases
end

set @end_time = GETDATE()

select DATEDIFF(millisecond, @start_time, @end_time) as execution_time_without_nolock

-- Loop 2: the execution time is bigger. Why?
select @start_time = GETDATE(), @from_id = 0
while @from_id < @to_id begin
	select top 1 @from_id += 1 from tmp_zs_456 with (nolock) -- Here is the execution time is bigger.
	--select top 1 @from_id += 1 from sys.databases with (nolock) -- This is ok.
end

set @end_time = GETDATE()

select DATEDIFF(millisecond, @start_time, @end_time) as execution_time_with_nolock