SQL Server - alternative to using NOT EXISTS - sql-server

Question

I have a list of about 200,000 records with EntityID column which I load into a temp table variable.
I want to insert any records from the Temp table variable if EntityID from the Temp table does not exist in the dbo.EntityRows table. The dbo.EntityRows table contains about 800,000 records.
The process is very slow compared to when the dbo.EntityRows table had about 500,000 records.
My first guess is because of the NOT EXISTS clause, each row from the Temp variable must scan the entire 800k rows of the dbo.EntityRows table to determine if it exists or not.
QUESTION: Are there alternative ways to run this comparison check without using the NOT EXISTS, which incurs a hefty cost and will only get worse as dbo.EntityRows continues to grow?
EDIT: Appreciate the comments. Here is the query (I left out the part after the IF NOT EXISTS check. After that, if NOT EXISTS, I insert into 4 tables).
declare #EntityCount int, #Counter int, #ExistsCounter int, #AddedCounter int
declare #LogID int
declare #YdataInsertedEntityID int, #YdataSearchParametersID int
declare #CurrentEntityID int
declare #CurrentName nvarchar(80)
declare #CurrentSearchParametersID int, #CurrentSearchParametersIDAlreadyDone int
declare #Entities table
(
Id int identity,
EntityID int,
NameID nvarchar(80),
SearchParametersID int
)
insert into #Entities
select EntityID, NameID, SearchParametersID from YdataArvixe.dbo.Entity order by entityid;
set #EntityCount = (select count(*) from #Entities);
set #Counter = 1;
set #LogID = null;
set #ExistsCounter = 0;
set #AddedCounter = 0;
set #CurrentSearchParametersIDAlreadyDone = -1;
While (#EntityCount >= #Counter)
begin
set #CurrentEntityID = (select EntityID from #Entities
where id = #Counter)
set #CurrentName = (select nameid from #Entities
where id = #Counter);
set #CurrentSearchParametersID = (select SearchParametersID from #Entities
where id = #Counter)
if not exists (select 1 from ydata.dbo.entity
where NameID = #CurrentName)
begin
-- I insert into 4 tables IF NOT EXISTS = true
end

Answer 1

I am not sure but there are following ways how we can check
(SELECT COUNT(er.EntityID) FROM dbo.EntityRows er WHERE er.EntityID = EntityID) <> 0
(SELECT er.EntityID FROM dbo.EntityRows er WHERE er.EntityID = EntityID) IS NOT NULL
EntityID NOT EXISTS (SELECT er.EntityID FROM dbo.EntityRows er)
EntityID NOT IN (SELECT er.EntityID FROM dbo.EntityRows er)
But as per my belief getting count will give good performance.
Also index will help to improve performance as 'Felix Pamittan' said

Answer 2

As #gotqn said, start by using a temporary table. Create an index on EntityID after the table is filled. If you don't have an index on EntityID in EntityRows, create one.
I do things like this a lot, and I generally use the following pattern:
INSERT INTO EntityRows (
EntityId, ...
)
SELECT T.EntityId, ...
FROM #tempTable T
LEFT JOIN EntityRows E
ON T.EntityID = E.EntityID
WHERE E.EntityID IS NULL
Please comment if you'd like further info.

Answer 3

Well, the answer was pretty basic. #Felix and #TT had the right suggestion. Thanks!
I put a non-clustered index on the NameID field in ydata.dbo.entity.
if not exists (select 1 from ydata.dbo.entity
where NameID = #CurrentName)
So it can now process the NOT EXISTS part quickly using the index instead of scanning the entire dbo.entity table. It is moving fast again.