Query slow after execute DBCC DROPCLEANBUFFERS - sql-server

Question

I have 2 tables Called "Users(60,0000 records),UserBasicInfo(60,0000 records)",The Users table have a clustered index on UsersID column.The UserBasicInfo table have a UsersID FK point to Users table UsersID column
and have Non-clustered on UpdateTime,UsersID column.
CREATE TABLE [dbo].[Users](
[UsersID] [int] IDENTITY(100000,1) NOT NULL,
[LoginUsersName] [nvarchar](50) NOT NULL,
[LoginUsersPwd] [nvarchar](50) NOT NULL,
[Email] [nvarchar](80) NOT NULL,
[IsEnable] [int] NOT NULL,
[CreateTime] [datetime] NOT NULL,
[LastLoginTime] [datetime] NOT NULL,
[LastLoginIp] [nvarchar](50) NOT NULL,
[UpdateTime] [datetime] NOT NULL,
CONSTRAINT [PK_Users] PRIMARY KEY CLUSTERED
(
[UsersID] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]
CREATE TABLE [dbo].[UserBasicInfo](
[UserBaicInfoID] [int] IDENTITY(1,1) NOT NULL,
[UsersID] [int] NOT NULL,
[ResumePoints] [nvarchar](50) NOT NULL,
[IsChineseOrEnglish] [int] NOT NULL,
[UserName] [nvarchar](50) NOT NULL,
[Sex] [int] NOT NULL,
[Height] [int] NOT NULL,
[Birthday] [datetime] NOT NULL,
[Age] [int] NOT NULL,
[IDCard] [nvarchar](50) NOT NULL,
[IsMarryed] [int] NOT NULL,
[NativePlace] [nvarchar](50) NOT NULL,
[PoliticalStatus] [int] NOT NULL,
[CurrentAddress] [nvarchar](50) NOT NULL,
[CurrentAddressDetail] [nvarchar](50) NOT NULL,
[WorkExperience] [int] NOT NULL,
[HighestEducation] [int] NOT NULL,
[LogoPath] [nvarchar](200) NULL,
[MobilePhone] [nvarchar](50) NOT NULL,
[Phone] [nvarchar](50) NULL,
[QQ] [nvarchar](50) NULL,
[Blog] [nvarchar](300) NULL,
[MicroBlog] [nvarchar](300) NULL,
[PositionDesired] [nvarchar](100) NOT NULL,
[IndustrySmallClass] [nvarchar](100) NULL,
[PositionName] [nvarchar](100) NOT NULL,
[PositionType] [int] NOT NULL,
[WorkAddressLarge] [int] NOT NULL,
[WorkAddressSmall] [nvarchar](200) NOT NULL,
[WorkAddressSmallText] [nvarchar](100) NOT NULL,
[Salary] [int] NOT NULL,
[HousingRequirement] [int] NOT NULL,
[ToWorkTime] [int] NOT NULL,
[ResumeState] [int] NOT NULL,
[IsSystemAdd] [int] NOT NULL,
[CreateTime] [datetime] NOT NULL,
[UpdateTime] [datetime] NOT NULL,
[RefreshDateTime] [datetime] NOT NULL,
[RefreshTime] [int] NOT NULL,
[TotalTime] [int] NOT NULL,
CONSTRAINT [PK_UserBasicInfo] PRIMARY KEY CLUSTERED
(
[UserBaicInfoID] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]
ALTER TABLE [dbo].[UserBasicInfo] WITH CHECK ADD CONSTRAINT [FK_UserBasicInfo_Users] FOREIGN KEY([UsersID])
REFERENCES [dbo].[Users] ([UsersID])
But execute below slow:
DBCC DROPCLEANBUFFERS
DBCC FREEPROCCACHE
SELECT TOP 100 * FROM Users U INNER JOIN UserBasicInfo UB ON UB.UsersID=U.UsersID ORDER BY UB.UpdateTime DESC
SQL Server Execution Times:
CPU time = 16 ms, elapsed time = 3568 ms.
Then execute below fast:
SELECT TOP 100 * FROM Users U INNER JOIN UserBasicInfo UB ON UB.UsersID=U.UsersID ORDER BY UB.UpdateTime DESC
SQL Server Execution Times:
CPU time = 0 ms, elapsed time = 163 ms.
It is normal?

Answer 1

Yes, it is normal
Your 2 DBCC commands ahve removed query plans and cached data. Data has to be read from disk again which is probably the main overhead.
If you add SET STATISTICS IO ON you'll see more "physical page reads" and "read-ahead reads" after the DBCC as data is loaded from disk
I almost never run them. For more, see these dba.se questions
https://dba.stackexchange.com/a/10820/630
https://dba.stackexchange.com/q/7859/630