diff --git a/Server.go b/Server.go
index 477b8e8..6744860 100644
--- a/Server.go
+++ b/Server.go
@@ -8,7 +8,6 @@ import (
 	"os"
 	"regexp"
 	"strings"
-	"sync"
 	"time"
 
 	"github.com/boltdb/bolt"
@@ -17,6 +16,8 @@ import (
 
 var SERVER_HEADER string = `contented/0.0.0-dev`
 
+const DEFAULT_MAX_CONCURRENT_THUMBS = 16
+
 type ServerPublicProperties struct {
 	AppTitle         string
 	MaxUploadBytes   int64
@@ -30,6 +31,7 @@ type ServerOptions struct {
 	BandwidthLimit         int64
 	TrustXForwardedFor     bool
 	EnableHomepage         bool
+	MaxConcurrentThumbs    int
 	ServerPublicProperties
 }
 
@@ -45,7 +47,7 @@ type Server struct {
 	opts           ServerOptions
 	db             *bolt.DB
 	startTime      time.Time
-	thumbnailSem   sync.Mutex
+	thumbnailSem   chan struct{}
 	metadataBucket []byte
 }
 
@@ -56,6 +58,17 @@ func NewServer(opts *ServerOptions) (*Server, error) {
 		startTime:      time.Now(),
 	}
 
+	if s.opts.MaxConcurrentThumbs <= 0 {
+		s.opts.MaxConcurrentThumbs = DEFAULT_MAX_CONCURRENT_THUMBS // default
+		log.Printf("Allowing %d concurrent thumbnails", s.opts.MaxConcurrentThumbs)
+	}
+
+	// "fill" the thumbnailer semaphore
+	s.thumbnailSem = make(chan struct{}, s.opts.MaxConcurrentThumbs)
+	for i := 0; i < s.opts.MaxConcurrentThumbs; i += 1 {
+		s.thumbnailSem <- struct{}{}
+	}
+
 	b, err := bolt.Open(opts.DBPath, 0644, bolt.DefaultOptions)
 	if err != nil {
 		return nil, err
diff --git a/cmd/contented/main.go b/cmd/contented/main.go
index d62c47c..9fdf4d6 100644
--- a/cmd/contented/main.go
+++ b/cmd/contented/main.go
@@ -21,6 +21,7 @@ func main() {
 	trustXForwardedFor := flag.Bool("trustXForwardedFor", false, "Trust X-Forwarded-For reverse proxy headers")
 	enableHomepage := flag.Bool("enableHomepage", true, "Enable homepage (disable for embedded use only)")
 	diskFilesWorldReadable := flag.Bool("diskFilesWorldReadable", false, "Save files as 0644 instead of 0600")
+	maxConcurrentThumbs := flag.Int("concurrentthumbs", contented.DEFAULT_MAX_CONCURRENT_THUMBS, "Simultaneous thumbnail generation")
 
 	flag.Parse()
 
@@ -31,6 +32,7 @@ func main() {
 		TrustXForwardedFor:     *trustXForwardedFor,
 		EnableHomepage:         *enableHomepage,
 		DiskFilesWorldReadable: *diskFilesWorldReadable,
+		MaxConcurrentThumbs:    *maxConcurrentThumbs,
 		ServerPublicProperties: contented.ServerPublicProperties{
 			AppTitle:       *appTitle,
 			MaxUploadBytes: int64(*maxUploadMb) * 1024 * 1024,
diff --git a/thumb.go b/thumb.go
index 556a208..b1bd24c 100644
--- a/thumb.go
+++ b/thumb.go
@@ -61,9 +61,9 @@ func (this *Server) handleThumb(w http.ResponseWriter, r *http.Request, thumbnai
 		return
 	}
 
-	// Only calculate one thumbnail at a time
-	this.thumbnailSem.Lock()
-	defer this.thumbnailSem.Unlock()
+	// Only a limited number of thumbnails can be generated concurrently
+	<-this.thumbnailSem
+	defer func() { this.thumbnailSem <- struct{}{} }()
 
 	if ctx.Err() != nil {
 		// The request was already cancelled