[Reporting] Fix performance of CSV generation (elastic#120309)

* [Reporting] Fix performance of CSV generation

* use a for loop with 1 operation instead of 3 chained maps

* do without the callback

* update comment

x-pack/plugins/reporting/server/export_types/csv_searchsource/generate_csv/generate_csv.ts

@@ -244,7 +244,7 @@ export class CsvGenerator {
   /*
    * Format a Datatable into rows of CSV content
    */
-  private generateRows(
+  private async generateRows(
     columns: string[],
     table: Datatable,
     builder: MaxSizeStringBuilder,
@@ -257,14 +257,38 @@ export class CsvGenerator {
         break;
       }
 
-      const row =
-        columns
-          .map((f) => ({ column: f, data: dataTableRow[f] }))
-          .map(this.formatCellValues(formatters))
-          .map(this.escapeValues(settings))
-          .join(settings.separator) + '\n';
+      /*
+       * Intrinsically, generating the rows is a synchronous process. Awaiting
+       * on a setImmediate call here partititions what could be a very long and
+       * CPU-intenstive synchronous process into an asychronous process. This
+       * give NodeJS to process other asychronous events that wait on the Event
+       * Loop.
+       *
+       * See: https://nodejs.org/en/docs/guides/dont-block-the-event-loop/
+       *
+       * It's likely this creates a lot of context switching, and adds to the
+       * time it would take to generate the CSV. There are alternatives to the
+       * chosen performance solution:
+       *
+       * 1. Partition the synchronous process with fewer partitions, by using
+       * the loop counter to call setImmediate only every N amount of rows.
+       * Testing is required to see what the best N value for most data will
+       * be.
+       *
+       * 2. Use a C++ add-on to generate the CSV using the Node Worker Pool
+       * instead of using the Event Loop
+       */
+      await new Promise(setImmediate);
+
+      const rowDefinition: string[] = [];
+      const format = this.formatCellValues(formatters);
+      const escape = this.escapeValues(settings);
+
+      for (const column of columns) {
+        rowDefinition.push(escape(format({ column, data: dataTableRow[column] })));
+      }
 
-      if (!builder.tryAppend(row)) {
+      if (!builder.tryAppend(rowDefinition.join(settings.separator) + '\n')) {
         this.logger.warn(`Max Size Reached after ${this.csvRowCount} rows.`);
         this.maxSizeReached = true;
         if (this.cancellationToken) {
@@ -379,7 +403,7 @@ export class CsvGenerator {
         }
 
         const formatters = this.getFormatters(table);
-        this.generateRows(columns, table, builder, formatters, settings);
+        await this.generateRows(columns, table, builder, formatters, settings);
 
         // update iterator
         currentRecord += table.rows.length;