diff --git a/internal/aggregate/metrics.go b/internal/aggregate/metrics.go
index 3742186..11b8e83 100644
--- a/internal/aggregate/metrics.go
+++ b/internal/aggregate/metrics.go
@@ -1,23 +1,33 @@
 package aggregate
 
 import (
+	"context"
+	"regexp"
+	"time"
+
 	"github.com/prometheus/client_golang/prometheus"
 	"notashelf.dev/watchdog/internal/config"
 )
 
+var prometheusLabelPattern = regexp.MustCompile(`^[a-zA-Z0-9_/:-]*$`)
+
 // Records analytics events as Prometheus metrics
 type MetricsAggregator struct {
-	registry     *PathRegistry
-	cfg          config.Config
-	pageviews    *prometheus.CounterVec
-	customEvents *prometheus.CounterVec
-	pathOverflow prometheus.Counter
-	dailyUniques prometheus.Gauge
-	estimator    *UniquesEstimator
+	pathRegistry  *PathRegistry
+	eventRegistry *CustomEventRegistry
+	cfg           config.Config
+	pageviews     *prometheus.CounterVec
+	customEvents  *prometheus.CounterVec
+	pathOverflow  prometheus.Counter
+	refOverflow   prometheus.Counter
+	eventOverflow prometheus.Counter
+	dailyUniques  prometheus.Gauge
+	estimator     *UniquesEstimator
+	stopChan      chan struct{}
 }
 
 // Creates a new metrics aggregator with dynamic labels based on config
-func NewMetricsAggregator(registry *PathRegistry, cfg config.Config) *MetricsAggregator {
+func NewMetricsAggregator(pathRegistry *PathRegistry, eventRegistry *CustomEventRegistry, cfg config.Config) *MetricsAggregator {
 	// Build label names based on what's enabled in config
 	labels := []string{"path"} // path is always included
 
@@ -56,6 +66,20 @@ func NewMetricsAggregator(registry *PathRegistry, cfg config.Config) *MetricsAgg
 		},
 	)
 
+	refOverflow := prometheus.NewCounter(
+		prometheus.CounterOpts{
+			Name: "web_referrer_overflow_total",
+			Help: "Referrers rejected due to cardinality limit",
+		},
+	)
+
+	eventOverflow := prometheus.NewCounter(
+		prometheus.CounterOpts{
+			Name: "web_event_overflow_total",
+			Help: "Custom events rejected due to cardinality limit",
+		},
+	)
+
 	dailyUniques := prometheus.NewGauge(
 		prometheus.GaugeOpts{
 			Name: "web_daily_unique_visitors",
@@ -63,40 +87,92 @@ func NewMetricsAggregator(registry *PathRegistry, cfg config.Config) *MetricsAgg
 		},
 	)
 
-	return &MetricsAggregator{
-		registry:     registry,
-		cfg:          cfg,
-		pageviews:    pageviews,
-		customEvents: customEvents,
-		pathOverflow: pathOverflow,
-		dailyUniques: dailyUniques,
-		estimator:    NewUniquesEstimator(),
+	m := &MetricsAggregator{
+		pathRegistry:  pathRegistry,
+		eventRegistry: eventRegistry,
+		cfg:           cfg,
+		pageviews:     pageviews,
+		customEvents:  customEvents,
+		pathOverflow:  pathOverflow,
+		refOverflow:   refOverflow,
+		eventOverflow: eventOverflow,
+		dailyUniques:  dailyUniques,
+		estimator:     NewUniquesEstimator(),
+		stopChan:      make(chan struct{}),
 	}
+
+	// Start background goroutine to update HLL gauge periodically
+	if cfg.Site.SaltRotation != "" {
+		go m.updateUniquesGauge()
+	}
+
+	return m
+}
+
+// Background goroutine to update the unique visitors gauge every 10 seconds
+// instead of on every request. This should help with performance.
+func (m *MetricsAggregator) updateUniquesGauge() {
+	ticker := time.NewTicker(10 * time.Second)
+	defer ticker.Stop()
+
+	for {
+		select {
+		case <-ticker.C:
+			estimate := m.estimator.Estimate()
+			m.dailyUniques.Set(float64(estimate))
+		case <-m.stopChan:
+			return
+		}
+	}
+}
+
+// Stop gracefully shuts down the background goroutine
+func (m *MetricsAggregator) Stop() {
+	close(m.stopChan)
+}
+
+// sanitizeLabel ensures label values are valid for Prometheus
+func sanitizeLabel(label string) string {
+	if !prometheusLabelPattern.MatchString(label) {
+		return "other"
+	}
+	if len(label) > 100 {
+		return "other"
+	}
+	return label
 }
 
 // Records a pageview with the configured dimensions
 func (m *MetricsAggregator) RecordPageview(path, country, device, referrer string) {
 	// Build label values in the same order as label names
-	labels := prometheus.Labels{"path": path}
+	labels := prometheus.Labels{"path": sanitizeLabel(path)}
 
 	if m.cfg.Site.Collect.Country {
-		labels["country"] = country
+		labels["country"] = sanitizeLabel(country)
 	}
 
 	if m.cfg.Site.Collect.Device {
-		labels["device"] = device
+		labels["device"] = sanitizeLabel(device)
 	}
 
 	if m.cfg.Site.Collect.Referrer != "off" {
-		labels["referrer"] = referrer
+		labels["referrer"] = sanitizeLabel(referrer)
 	}
 
 	m.pageviews.With(labels).Inc()
 }
 
-// Records a custom event
+// Records a custom event with cardinality protection
 func (m *MetricsAggregator) RecordCustomEvent(eventName string) {
-	m.customEvents.With(prometheus.Labels{"event": eventName}).Inc()
+	// Use registry to enforce cardinality limit
+	sanitized := sanitizeLabel(eventName)
+	accepted := m.eventRegistry.Add(sanitized)
+
+	if accepted == "other" {
+		m.eventOverflow.Inc()
+	}
+
+	m.customEvents.With(prometheus.Labels{"event": accepted}).Inc()
 }
 
 // Records a path that was rejected due to cardinality limits
@@ -104,14 +180,20 @@ func (m *MetricsAggregator) RecordPathOverflow() {
 	m.pathOverflow.Inc()
 }
 
-// Adds a visitor to the unique visitor estimator. Only tracks if salt_rotation is configured
+// Records a referrer that was rejected due to cardinality limits
+func (m *MetricsAggregator) RecordReferrerOverflow() {
+	m.refOverflow.Inc()
+}
+
+// Adds a visitor to the unique visitor estimator.
+// Only tracks if salt_rotation is configured
 func (m *MetricsAggregator) AddUnique(ip, userAgent string) {
 	if m.cfg.Site.SaltRotation == "" {
 		return // only track if salt rotation is enabled
 	}
 
 	m.estimator.Add(ip, userAgent)
-	m.dailyUniques.Set(float64(m.estimator.Estimate()))
+	// Note: Gauge is updated in background goroutine, not here
 }
 
 // Registers all metrics with the provided Prometheus registry
@@ -119,5 +201,17 @@ func (m *MetricsAggregator) MustRegister(reg prometheus.Registerer) {
 	reg.MustRegister(m.pageviews)
 	reg.MustRegister(m.customEvents)
 	reg.MustRegister(m.pathOverflow)
+	reg.MustRegister(m.refOverflow)
+	reg.MustRegister(m.eventOverflow)
 	reg.MustRegister(m.dailyUniques)
 }
+
+// Shutdown performs graceful shutdown operations
+func (m *MetricsAggregator) Shutdown(ctx context.Context) error {
+	m.Stop()
+	// Persist HLL state if configured
+	if m.cfg.Site.SaltRotation != "" {
+		return m.estimator.Save("/tmp/watchdog-hll.state")
+	}
+	return nil
+}
diff --git a/internal/aggregate/uniques.go b/internal/aggregate/uniques.go
index b1049e0..74ec2da 100644
--- a/internal/aggregate/uniques.go
+++ b/internal/aggregate/uniques.go
@@ -1,8 +1,11 @@
 package aggregate
 
 import (
+	"bytes"
 	"crypto/sha256"
 	"encoding/hex"
+	"fmt"
+	"os"
 	"sync"
 	"time"
 
@@ -77,3 +80,48 @@ func (u *UniquesEstimator) CurrentSalt() string {
 func DailySalt(t time.Time) string {
 	return dailySalt(t)
 }
+
+// Save persists the HLL state to disk
+func (u *UniquesEstimator) Save(path string) error {
+	u.mu.Lock()
+	defer u.mu.Unlock()
+
+	data, err := u.hll.MarshalBinary()
+	if err != nil {
+		return err
+	}
+
+	// Save both HLL data and current day salt
+	return os.WriteFile(path, append([]byte(u.currentDay+"\n"), data...), 0600)
+}
+
+// Load restores the HLL state from disk
+func (u *UniquesEstimator) Load(path string) error {
+	data, err := os.ReadFile(path)
+	if err != nil {
+		return err // File not existing is OK (first run)
+	}
+
+	u.mu.Lock()
+	defer u.mu.Unlock()
+
+	// Parse saved salt and HLL data
+	parts := bytes.SplitN(data, []byte("\n"), 2)
+	if len(parts) != 2 {
+		return fmt.Errorf("invalid state file format")
+	}
+
+	savedSalt := string(parts[0])
+	today := dailySalt(time.Now())
+
+	// Only restore if it's the same day
+	if savedSalt == today {
+		u.currentDay = savedSalt
+		return u.hll.UnmarshalBinary(parts[1])
+	}
+
+	// Different day - start fresh
+	u.hll = hyperloglog.New()
+	u.currentDay = today
+	return nil
+}