diff --git a/src/timezone/README b/src/timezone/README
index 2544230c4c..912e0c1b39 100644
--- a/src/timezone/README
+++ b/src/timezone/README
@@ -79,6 +79,9 @@ other exposed names.
 slightly modified the API of the former, in part because it now relies
 on our own pg_open_tzfile() rather than opening files for itself.
 
+* tzparse() is adjusted to cache the result of loading the TZDEFRULES
+zone, so that that's not repeated more than once per process.
+
 * There's a fair amount of code we don't need and have removed,
 including all the nonstandard optional APIs.  We have also added
 a few functions of our own at the bottom of localtime.c.
diff --git a/src/timezone/localtime.c b/src/timezone/localtime.c
index 6e0fa10066..04105b2c96 100644
--- a/src/timezone/localtime.c
+++ b/src/timezone/localtime.c
@@ -54,6 +54,13 @@ static const char gmt[] = "GMT";
 static const pg_time_t time_t_min = MINVAL(pg_time_t, TYPE_BIT(pg_time_t));
 static const pg_time_t time_t_max = MAXVAL(pg_time_t, TYPE_BIT(pg_time_t));
 
+/*
+ * We cache the result of trying to load the TZDEFRULES zone here.
+ * tzdefrules_loaded is 0 if not tried yet, +1 if good, -1 if failed.
+ */
+static struct state tzdefrules_s;
+static int	tzdefrules_loaded = 0;
+
 /*
  * The DST rules to use if TZ has no rules and we can't load TZDEFRULES.
  * We default to US rules as of 1999-08-17.
@@ -942,7 +949,21 @@ tzparse(const char *name, struct state * sp, bool lastditch)
 		charcnt = stdlen + 1;
 		if (sizeof sp->chars < charcnt)
 			return false;
-		load_ok = tzload(TZDEFRULES, NULL, sp, false) == 0;
+
+		/*
+		 * This bit also differs from the IANA code, which doesn't make any
+		 * attempt to avoid repetitive loadings of the TZDEFRULES zone.
+		 */
+		if (tzdefrules_loaded == 0)
+		{
+			if (tzload(TZDEFRULES, NULL, &tzdefrules_s, false) == 0)
+				tzdefrules_loaded = 1;
+			else
+				tzdefrules_loaded = -1;
+		}
+		load_ok = (tzdefrules_loaded > 0);
+		if (load_ok)
+			memcpy(sp, &tzdefrules_s, sizeof(struct state));
 	}
 	if (!load_ok)
 		sp->leapcnt = 0;		/* so, we're off a little */
diff --git a/src/timezone/pgtz.c b/src/timezone/pgtz.c
index 982fc1fe71..9356f2592d 100644
--- a/src/timezone/pgtz.c
+++ b/src/timezone/pgtz.c
@@ -80,12 +80,37 @@ pg_open_tzfile(const char *name, char *canonname)
 	int			fullnamelen;
 	int			orignamelen;
 
+	/* Initialize fullname with base name of tzdata directory */
+	strlcpy(fullname, pg_TZDIR(), sizeof(fullname));
+	orignamelen = fullnamelen = strlen(fullname);
+
+	if (fullnamelen + 1 + strlen(name) >= MAXPGPATH)
+		return -1;				/* not gonna fit */
+
+	/*
+	 * If the caller doesn't need the canonical spelling, first just try to
+	 * open the name as-is.  This can be expected to succeed if the given name
+	 * is already case-correct, or if the filesystem is case-insensitive; and
+	 * we don't need to distinguish those situations if we aren't tasked with
+	 * reporting the canonical spelling.
+	 */
+	if (canonname == NULL)
+	{
+		int			result;
+
+		fullname[fullnamelen] = '/';
+		/* test above ensured this will fit: */
+		strcpy(fullname + fullnamelen + 1, name);
+		result = open(fullname, O_RDONLY | PG_BINARY, 0);
+		if (result >= 0)
+			return result;
+		/* If that didn't work, fall through to do it the hard way */
+	}
+
 	/*
 	 * Loop to split the given name into directory levels; for each level,
 	 * search using scan_directory_ci().
 	 */
-	strlcpy(fullname, pg_TZDIR(), sizeof(fullname));
-	orignamelen = fullnamelen = strlen(fullname);
 	fname = name;
 	for (;;)
 	{
@@ -97,8 +122,6 @@ pg_open_tzfile(const char *name, char *canonname)
 			fnamelen = slashptr - fname;
 		else
 			fnamelen = strlen(fname);
-		if (fullnamelen + 1 + fnamelen >= MAXPGPATH)
-			return -1;			/* not gonna fit */
 		if (!scan_directory_ci(fullname, fname, fnamelen,
 							   fullname + fullnamelen + 1,
 							   MAXPGPATH - fullnamelen - 1))
@@ -458,10 +481,11 @@ pg_tzenumerate_next(pg_tzenum *dir)
 
 		/*
 		 * Load this timezone using tzload() not pg_tzset(), so we don't fill
-		 * the cache
+		 * the cache.  Also, don't ask for the canonical spelling: we already
+		 * know it, and pg_open_tzfile's way of finding it out is pretty
+		 * inefficient.
 		 */
-		if (tzload(fullname + dir->baselen, dir->tz.TZname, &dir->tz.state,
-				   true) != 0)
+		if (tzload(fullname + dir->baselen, NULL, &dir->tz.state, true) != 0)
 		{
 			/* Zone could not be loaded, ignore it */
 			continue;
@@ -473,6 +497,10 @@ pg_tzenumerate_next(pg_tzenum *dir)
 			continue;
 		}
 
+		/* OK, return the canonical zone name spelling. */
+		strlcpy(dir->tz.TZname, fullname + dir->baselen,
+				sizeof(dir->tz.TZname));
+
 		/* Timezone loaded OK. */
 		return &dir->tz;
 	}