[FIX] spreadsheet: mark pivot cell faster

In HBA's demo sheet, `markAsValueUsed` goes from more than 2% of the total
loading time to less than 1%

closes odoo/odoo#138166

Signed-off-by: Rémi Rahir (rar) <rar@odoo.com>
Signed-off-by: Lucas Lefèvre (lul) <lul@odoo.com>

addons/spreadsheet/static/src/pivot/pivot_model.js

@@ -216,8 +216,7 @@ export class SpreadsheetPivotModel extends PivotModel {
      * Check if the given domain with the given measure has been used
      */
     isUsedValue(domain, measure) {
-        const tag = [measure, ...domain];
-        return this._usedValueDomains.has(tag.join());
+        return this._usedValueDomains.has(measure + "," + domain.join());
     }
 
     /**
@@ -231,8 +230,7 @@ export class SpreadsheetPivotModel extends PivotModel {
      * Indicate that the given domain has been used with the given measure
      */
     markAsValueUsed(domain, measure) {
-        const toTag = [measure, ...domain];
-        this._usedValueDomains.add(toTag.join());
+        this._usedValueDomains.add(measure + "," + domain.join());
     }
 
     /**