diff --git a/internal/api/event.go b/internal/api/event.go
index e4dfe88..7cb828f 100644
--- a/internal/api/event.go
+++ b/internal/api/event.go
@@ -4,6 +4,7 @@ import (
 	"encoding/json"
 	"fmt"
 	"io"
+	"slices"
 
 	"notashelf.dev/watchdog/internal/limits"
 )
@@ -46,13 +47,7 @@ func (e *Event) Validate(allowedDomains []string) error {
 	}
 
 	// Check if domain is in allowed list
-	allowed := false
-	for _, domain := range allowedDomains {
-		if e.Domain == domain {
-			allowed = true
-			break
-		}
-	}
+	allowed := slices.Contains(allowedDomains, e.Domain)
 	if !allowed {
 		return fmt.Errorf("domain not allowed")
 	}
@@ -76,3 +71,34 @@ func (e *Event) Validate(allowedDomains []string) error {
 
 	return nil
 }
+
+// ValidateWithMap checks if the event is valid using a domain map (O(1) lookup)
+func (e *Event) ValidateWithMap(allowedDomains map[string]bool) error {
+	if e.Domain == "" {
+		return fmt.Errorf("domain required")
+	}
+
+	// Check if domain is in allowed map (O(1) instead of O(n))
+	if !allowedDomains[e.Domain] {
+		return fmt.Errorf("domain not allowed")
+	}
+
+	if e.Path == "" {
+		return fmt.Errorf("path required")
+	}
+
+	if len(e.Path) > limits.MaxPathLen {
+		return fmt.Errorf("path too long")
+	}
+
+	if len(e.Referrer) > limits.MaxRefLen {
+		return fmt.Errorf("referrer too long")
+	}
+
+	// Validate screen width is in reasonable range
+	if e.Width < 0 || e.Width > limits.MaxWidth {
+		return fmt.Errorf("invalid width")
+	}
+
+	return nil
+}
diff --git a/internal/api/event_test.go b/internal/api/event_test.go
index f9d0dcf..0a075fe 100644
--- a/internal/api/event_test.go
+++ b/internal/api/event_test.go
@@ -230,3 +230,37 @@ func TestValidateEvent(t *testing.T) {
 		})
 	}
 }
+
+func BenchmarkValidate_SliceLookup(b *testing.B) {
+	// Simulate multi-site with 50 domains
+	domains := make([]string, 50)
+	for i := range 50 {
+		domains[i] = strings.Repeat("site", i) + ".com"
+	}
+
+	event := Event{
+		Domain: domains[49], // Worst case - last in list
+		Path:   "/test",
+	}
+
+	for b.Loop() {
+		_ = event.Validate(domains)
+	}
+}
+
+func BenchmarkValidate_MapLookup(b *testing.B) {
+	// Simulate multi-site with 50 domains
+	domainMap := make(map[string]bool, 50)
+	for i := range 50 {
+		domainMap[strings.Repeat("site", i)+".com"] = true
+	}
+
+	event := Event{
+		Domain: strings.Repeat("site", 49) + ".com", // any position
+		Path:   "/test",
+	}
+
+	for b.Loop() {
+		_ = event.ValidateWithMap(domainMap)
+	}
+}
diff --git a/internal/api/handler.go b/internal/api/handler.go
index 049b2e7..bed0abf 100644
--- a/internal/api/handler.go
+++ b/internal/api/handler.go
@@ -15,6 +15,7 @@ import (
 // Handles incoming analytics events
 type IngestionHandler struct {
 	cfg          *config.Config
+	domainMap    map[string]bool // O(1) domain validation
 	pathNorm     *normalize.PathNormalizer
 	pathRegistry *aggregate.PathRegistry
 	refRegistry  *normalize.ReferrerRegistry
@@ -40,8 +41,15 @@ func NewIngestionHandler(
 		)
 	}
 
+	// Build domain map for O(1) validation
+	domainMap := make(map[string]bool, len(cfg.Site.Domains))
+	for _, domain := range cfg.Site.Domains {
+		domainMap[domain] = true
+	}
+
 	return &IngestionHandler{
 		cfg:          cfg,
+		domainMap:    domainMap,
 		pathNorm:     pathNorm,
 		pathRegistry: pathRegistry,
 		refRegistry:  refRegistry,
@@ -95,8 +103,8 @@ func (h *IngestionHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	// Validate event
-	if err := event.Validate(h.cfg.Site.Domains); err != nil {
+	// Validate event via map lookup (also O(1))
+	if err := event.ValidateWithMap(h.domainMap); err != nil {
 		http.Error(w, "Bad request", http.StatusBadRequest)
 		return
 	}