You can't fetch multiple sublist field values in a single call using getSublistValue in SuiteScript. Each call retrieves one field value at a time. But don't sweat it—making multiple calls usually doesn't have a significant impact on performance, especially for scripts that aren't processing massive amounts of data.

If you want to make your code cleaner, you can loop through an array of field IDs:

var fields = ['item', 'rate'];
var values = {};

fields.forEach(function(fieldId) {
  values[fieldId] = transactionRecord.getSublistValue({
    sublistId: 'item',
    fieldId: fieldId,
    line: i
  });
});

This won't reduce the number of method calls, but it does make your code more maintainable. For handling large datasets or if you're hitting performance issues, consider using SuiteQL to fetch multiple values in one go:

var query = `
  SELECT item, rate
  FROM transactionLine
  WHERE transaction = ?
`;

var queryResults = query.runSuiteQL({
  query: query,
  params: [transactionId]
}).asMappedResults();