unresponsive script problem

I am having 5000 records in my grid. In my grid i am trying to use multiple selection . When the time of click the header check box , i am trying to select
that 5000 records check box's. So when the time of doing that
i am facing the unresponsive script problem.

I think that setCellValue() had problem. In this setCellValue () i am passing the check box value ( true) and column number and row number.
num_rowss=5000;
Eg:
for (i=0; i<num_rows; i++){
obj.setCellValue('true', 1, i);
}
When the time of run my program i got that problem.

Can any one explain other way to select the check box
(or)
setCellValue() had problem or not .
It is very very urgent ......

Alex, kindly look in to this problem and give me solution.