I am trying to create a script that when a custom button is selected on specific Cases, it looks through a sublist of custom records (which holds item, quantity, price, and other info) and creates a Quote using those custom records to populate the line items. I have been trying to perform this so when the button is clicked a function is performed from a client script in the file cabinet, but anything more than 5 items can take more than 30 seconds to load, and causes the browser to show NetSuite as unresponsive, which is not what my requester wants. Am I going about this in the right way? Is there a way to avoid the long load time or have it work in the backend so the user can still work? I am newer to SuiteScript so any assistance is appreciated! (Code in comments)

Depends on the script and what it is going. Sharing your code would likely help with responses.

For sure, here is the code for the button:

const beforeLoad = (context) => {
            return logErrorIfNeeded("beforeLoad", function() {
                var currentRecord = context.newRecord;
    
                context.form.addButton({
                    id: 'custpage_quote_button',
                    label: 'Create Quote',
                    functionName: 'createQuoteForm'
                });
                context.form.clientScriptModulePath = "SuiteScripts/LX_CL_Quote_Line_Items.js";
            });
        };
        
        return {beforeLoad}

And the code for file cabinet client script:

function createQuoteForm() {
            return logErrorIfNeeded("pageInit", function () {
                var currentRecObj = currentRecordMod.get();
                var caseId = currentRecObj.id;
                var caseRecord = record.load({
                    type: record.Type.SUPPORT_CASE,
                    id: caseId
                });
                
                var quoteRecord = record.create({
                    type: record.Type.ESTIMATE,
                    isDynamic: true
                });

                //set fields on quote
                quoteRecord.setValue({
                    fieldId: 'entity',
                    value: caseRecord.getValue({fieldId: 'company'})
                });
                ...
                ...
                ...

                var sublistName = 'recmachcustrecord_na_case';

                var itemLineCount = caseRecord.getLineCount({
                    sublistId: sublistName
                });

                log.debug("itemLineCount", itemLineCount);

                for (var i = 0; i < itemLineCount; i++) {

                    //get item fields from case
                    var itemSKU = caseRecord.getSublistValue({
                        sublistId: sublistName,
                        fieldId: 'custrecord_na_item',
                        line: i
                    });
                    ...
                    ...
                    ...

                    //add line
                    quoteRecord.selectNewLine({sublistId: 'item'});

                    //set line item fields
                    quoteRecord.setCurrentSublistValue({sublistId: "item", fieldId: "item", value: itemSKU, forceSyncSourcing: true});
                    ...
                    ...
                    ...

                    //commit line
                    quoteRecord.commitLine({sublistId: 'item'});
                }

                var quoteId = quoteRecord.save();

                var quoteUrl = url.resolveRecord({
                    recordType: 'estimate',
                    recordId: quoteId,
                    isEditMode: true
                });

                window.open(quoteUrl);
            })
                
        }

        return {
            pageInit: pageInit,
            createQuoteForm: createQuoteForm
        };

Hmm, ok so I am by no means an expert but I think you'll be somewhat limited with a client script. If you don't need it real-time I would say N/task but I don't think that is exposed to client scripts. So my next thought would be transfer all the record create logic to a suitelet and just pass the caseId to the suitelet as a param with redirect.toSuitelet or pass it in the URL with &caseid=1234 and grab it in the suitelet with context.request.parameters.caseid, do all the stuff then redirect to the new record when done. Or you could trigger N/task here if a background task is sufficient.

Thank you! I found a thread that mentioned going that route but wanted to be sure - I'll try it out and see what's up. Appreciate the response!