Nested loops have their place in development, but if you aren't careful, they can lead to serious performance degradation due to the fact that the inner loop will run a certain number of times, multiplied by the number of times the outer loop runs.

It is often possible to avoid having to run a nested loop, in favor of running two separate loops (which means that each loop would only run a set number of times).

Consider the following code:

var innerGR; 
var results = [];  
var outerGR = new GlideRecord('table_name'); 
outerGR.query(); 
while (outerGR.next()) { 
    innerGR = new GlideRecord('other_table_name'); 
    innerGR.addQuery('field', outerGR.getValue('something_to_compare')); 
    innerGR.query(); 
    while (innerGR.next()) { 
        results.push(innerGR.getValue('some_field')); 
    } 
} 

This code requires constructing a new GlideRecord object (innerGR), a new query, and iterating over each record - for every record found within outerGR. The only reason it's constructed this way, is because we want to get a value from records in innerGR that meet a query based on another value from outerGR, and push them into results. However, we can accomplish this much more efficiently by simply separating out these two queries, and creating a temporary array to store up the values from outerGR, that we're using in the query in innerGR like so:

var queryValues = []; 
var results = []; 

var outerGR = new GlideRecord('table_name'); 
outerGR.query(); 
while (outerGR.next()) { 
    queryValues.push(outerGR.getValue('something_to_compare')); 
}  
innerGR = new GlideRecord('other_table_name'); 
innerGR.addQuery('field', 'IN', queryValues); 
innerGR.query(); 
while (innerGR.next()) { 
    results.push(innerGR.getValue('some_field')); 
} 

As you can see, by pushing the values we want to search on into an array, and then performing only one outer (though no longer outer, but stand-alone) query, we can dramatically improve performance of our script.