ADS 11.10.0.24 Performance Issue

I am having an issue with a script that I am running. In the example below, it takes a lot longer to run the query if @InputAccountNumber is equal to 1 instead of null, when I would expect the query to filter the records and be significantly faster.  In contrast, if I set the @InputPostingDate value to be today I don’t see the same slowness issue. 

//Create some test data
DECLARE i integer;

TRY
createtable#TEMP ( AccountNumber integer, PostingDate date);
createindex AccountNumber on#TEMP(AccountNumber);
createindex PostingDate on#TEMP(PostingDate);
CATCHALL
deletefrom#TEMP;
ENDTRY;

insertinto#TEMP ( AccountNumber, PostingDate )
values( 1, CURDATE());

i = 1;

while i < 10 do
insertinto#TEMP ( AccountNumber, PostingDate )
select AccountNumber + 1, PostingDate + 1
from#TEMP;

i = i + 1;
endwhile;

selectcount(*)from#TEMP


//Run the script to test the issue

DECLARE@InputAccountNumber integer;
DECLARE@InputPostingDate date;

@InputPostingDate =null;
@InputAccountNumber = 1;

select*
from#TEMP
where(@InputAccountNumber isnullor AccountNumber =@InputAccountNumber )
and(@InputPostingDate isnullor PostingDate <=@InputPostingDate )