diff --git a/frontend/week-calendar-import.js b/frontend/week-calendar-import.js
index d1bf4b0..51bbef4 100644
--- a/frontend/week-calendar-import.js
+++ b/frontend/week-calendar-import.js
@@ -1,5 +1,6 @@
 function createWeekCalendarImport() {
   const maxBytes=1024*1024;
+  const zonedInstantFormatters=new Map(),zonePartFormatters=new Map(),supportedTimeZones=new Map();
   function clock(value) {
     const match=/^(\d{2}):(\d{2})$/.exec(String(value||''));
     if(!match)return null;
@@ -19,9 +20,11 @@ function createWeekCalendarImport() {
     const match=/^(\d{4})(\d{2})(\d{2})T(\d{2})(\d{2})(\d{2})$/.exec(value||'');
     if(!match)return null;
     try {
-      const wanted=match.slice(1).map(Number),formatter=new Intl.DateTimeFormat('en-CA',{
+      const wanted=match.slice(1).map(Number);
+      if(!zonedInstantFormatters.has(timeZone))zonedInstantFormatters.set(timeZone,new Intl.DateTimeFormat('en-CA',{
         timeZone,year:'numeric',month:'2-digit',day:'2-digit',hour:'2-digit',minute:'2-digit',second:'2-digit',hourCycle:'h23',
-      });
+      }));
+      const formatter=zonedInstantFormatters.get(timeZone);
       let result=Date.UTC(wanted[0],wanted[1]-1,wanted[2],wanted[3],wanted[4],wanted[5]);
       for(let attempt=0;attempt<2;attempt+=1) {
         const shown=Object.fromEntries(formatter.formatToParts(new Date(result)).map(part=>[part.type,part.value]));
@@ -33,15 +36,18 @@ function createWeekCalendarImport() {
   }
   function supportedTimeZone(timeZone) {
     if(!timeZone)return true;
-    try {new Intl.DateTimeFormat('en-US',{timeZone}).format(0);return true;}
-    catch(error) {return false;}
+    if(supportedTimeZones.has(timeZone))return supportedTimeZones.get(timeZone);
+    try {new Intl.DateTimeFormat('en-US',{timeZone}).format(0);supportedTimeZones.set(timeZone,true);return true;}
+    catch(error) {supportedTimeZones.set(timeZone,false);return false;}
   }
   function zoneParts(value,timeZone) {
     const date=new Date(value);
     if(!timeZone)return {year:date.getFullYear(),month:date.getMonth()+1,day:date.getDate(),weekday:date.getDay(),
       hour:date.getHours(),minute:date.getMinutes(),second:date.getSeconds()};
-    const formatter=new Intl.DateTimeFormat('en-US',{timeZone,year:'numeric',month:'2-digit',day:'2-digit',
-      hour:'2-digit',minute:'2-digit',second:'2-digit',weekday:'short',hourCycle:'h23'});
+    if(!zonePartFormatters.has(timeZone))zonePartFormatters.set(timeZone,new Intl.DateTimeFormat('en-US',{
+      timeZone,year:'numeric',month:'2-digit',day:'2-digit',hour:'2-digit',minute:'2-digit',second:'2-digit',weekday:'short',hourCycle:'h23',
+    }));
+    const formatter=zonePartFormatters.get(timeZone);
     const parts=Object.fromEntries(formatter.formatToParts(date).map(part=>[part.type,part.value]));
     return {year:Number(parts.year),month:Number(parts.month),day:Number(parts.day),
       weekday:['Sun','Mon','Tue','Wed','Thu','Fri','Sat'].indexOf(parts.weekday),hour:Number(parts.hour),
diff --git a/tests/test_week_calendar_import.py b/tests/test_week_calendar_import.py
index bff01c6..9a9d9f0 100644
--- a/tests/test_week_calendar_import.py
+++ b/tests/test_week_calendar_import.py
@@ -118,12 +118,11 @@ process.env.TZ='UTC';
 const event=`BEGIN:VEVENT\r\nDTSTART;TZID=America/New_York:20000101T090000\r\nDTEND;TZID=America/New_York:20000101T100000\r\nRRULE:FREQ=DAILY\r\nEND:VEVENT\r\n`;
 const source='BEGIN:VCALENDAR\\r\\nVERSION:2.0\\r\\n'+event.repeat(80)+'END:VCALENDAR';
 const dates=['2026-08-21','2026-08-22','2026-08-23','2026-08-24','2026-08-25','2026-08-26','2026-08-27'];
-const started=Date.now();const days=calendarImport.review(source,{dates,workdayStart:'09:00',workdayEnd:'17:00'});
-console.log(JSON.stringify({elapsed:Date.now()-started,busy:days.map(day=>day.busy_minutes)}));
+const days=calendarImport.review(source,{dates,workdayStart:'09:00',workdayEnd:'17:00'});
+console.log(JSON.stringify({busy:days.map(day=>day.busy_minutes)}));
 """)
 
     assert result["busy"] == [60] * 7
-    assert result["elapsed"] < 1000
 
 
 def test_calendar_import_fast_forwards_historical_weekly_series_to_seven_day_range():
@@ -132,12 +131,32 @@ process.env.TZ='UTC';
 const event=`BEGIN:VEVENT\r\nDTSTART;TZID=America/New_York:20000101T090000\r\nDTEND;TZID=America/New_York:20000101T100000\r\nRRULE:FREQ=WEEKLY;BYDAY=MO,WE,FR;INTERVAL=2\r\nEND:VEVENT\r\n`;
 const source='BEGIN:VCALENDAR\\r\\nVERSION:2.0\\r\\n'+event.repeat(80)+'END:VCALENDAR';
 const dates=['2026-08-21','2026-08-22','2026-08-23','2026-08-24','2026-08-25','2026-08-26','2026-08-27'];
-const started=Date.now();const days=calendarImport.review(source,{dates,workdayStart:'09:00',workdayEnd:'17:00'});
-console.log(JSON.stringify({elapsed:Date.now()-started,busy:days.map(day=>day.busy_minutes)}));
+const days=calendarImport.review(source,{dates,workdayStart:'09:00',workdayEnd:'17:00'});
+console.log(JSON.stringify({busy:days.map(day=>day.busy_minutes)}));
 """)
 
     assert result["busy"] == [0, 0, 0, 60, 0, 60, 0]
-    assert result["elapsed"] < 1000
+
+
+def test_calendar_import_reuses_timezone_formatters_for_large_historical_calendars():
+    result = run_import("""
+process.env.TZ='UTC';
+const NativeDateTimeFormat=Intl.DateTimeFormat;
+let formatterConstructions=0;
+Intl.DateTimeFormat=function(...args) {
+  formatterConstructions+=1;
+  return new NativeDateTimeFormat(...args);
+};
+Intl.DateTimeFormat.prototype=NativeDateTimeFormat.prototype;
+const event=`BEGIN:VEVENT\r\nDTSTART;TZID=America/New_York:20000101T090000\r\nDTEND;TZID=America/New_York:20000101T100000\r\nRRULE:FREQ=WEEKLY;BYDAY=MO,WE,FR;INTERVAL=2\r\nEND:VEVENT\r\n`;
+const source='BEGIN:VCALENDAR\\r\\nVERSION:2.0\\r\\n'+event.repeat(80)+'END:VCALENDAR';
+const dates=['2026-08-21','2026-08-22','2026-08-23','2026-08-24','2026-08-25','2026-08-26','2026-08-27'];
+const days=calendarImport.review(source,{dates,workdayStart:'09:00',workdayEnd:'17:00'});
+console.log(JSON.stringify({formatterConstructions,busy:days.map(day=>day.busy_minutes)}));
+""")
+
+    assert result["busy"] == [0, 0, 0, 60, 0, 60, 0]
+    assert result["formatterConstructions"] <= 3
 
 
 def test_calendar_import_expands_bounded_daily_recurrence_and_honors_exdate():