diff --git a/cbconvert.go b/cbconvert.go
index 4523d03..5b8ac42 100644
--- a/cbconvert.go
+++ b/cbconvert.go
@@ -25,6 +25,8 @@ type Options struct {
 	Archive string
 	// JPEG image quality
 	Quality int
+	// Encoder speed/effort, format-specific: webp method 0-6, avif speed 0-10, jxl effort 1-10; -1 uses the format default
+	Effort int
 	// Image width
 	Width int
 	// Image height
@@ -125,6 +127,7 @@ func NewOptions() Options {
 	o.Format = "jpeg"
 	o.Archive = "zip"
 	o.Quality = 75
+	o.Effort = -1
 	o.Filter = 2
 
 	return o
diff --git a/cbconvert_convert.go b/cbconvert_convert.go
index 1f5fd29..b1f097e 100644
--- a/cbconvert_convert.go
+++ b/cbconvert_convert.go
@@ -398,11 +398,23 @@ func (c *Converter) imageEncode(img image.Image, w io.Writer) error {
 		opts.DCTMethod = jpegli.DefaultDCTMethod
 		err = jpegli.Encode(w, img, opts)
 	case "webp":
-		err = webp.Encode(w, img, webp.Options{Quality: c.Opts.Quality, Method: webp.DefaultMethod})
+		method := webp.DefaultMethod
+		if c.Opts.Effort >= 0 {
+			method = min(max(c.Opts.Effort, 0), 6)
+		}
+		err = webp.Encode(w, img, webp.Options{Quality: c.Opts.Quality, Method: method})
 	case "avif":
-		err = avif.Encode(w, img, avif.Options{Quality: c.Opts.Quality, Speed: avif.DefaultSpeed})
+		speed := avif.DefaultSpeed
+		if c.Opts.Effort >= 0 {
+			speed = min(max(c.Opts.Effort, 0), 10)
+		}
+		err = avif.Encode(w, img, avif.Options{Quality: c.Opts.Quality, Speed: speed})
 	case "jxl":
-		err = jpegxl.Encode(w, img, jpegxl.Options{Quality: c.Opts.Quality, Effort: jpegxl.DefaultEffort})
+		effort := jpegxl.DefaultEffort
+		if c.Opts.Effort >= 0 {
+			effort = min(max(c.Opts.Effort, 1), 10)
+		}
+		err = jpegxl.Encode(w, img, jpegxl.Options{Quality: c.Opts.Quality, Effort: effort})
 	case "bmp":
 		opts := &gobmp.EncoderOptions{}
 		opts.SupportTransparency(false)
diff --git a/cmd/cbconvert-gui/main.go b/cmd/cbconvert-gui/main.go
index e11af0b..e3b2927 100644
--- a/cmd/cbconvert-gui/main.go
+++ b/cmd/cbconvert-gui/main.go
@@ -140,6 +140,12 @@ func options() cbconvert.Options {
 	opts.Fit = iup.GetHandle("Fit").GetAttribute("VALUE") == "ON"
 	opts.Filter = iup.GetHandle("Filter").GetInt("VALUE") - 1
 	opts.Quality = iup.GetHandle("Quality").GetInt("VALUE")
+	switch opts.Format {
+	case "webp", "avif", "jxl":
+		opts.Effort = iup.GetHandle("Effort").GetInt("VALUE")
+	default:
+		opts.Effort = -1
+	}
 	opts.Grayscale = iup.GetHandle("Grayscale").GetAttribute("VALUE") == "ON"
 	opts.Brightness = iup.GetHandle("Brightness").GetInt("VALUE")
 	opts.Contrast = iup.GetHandle("Contrast").GetInt("VALUE")
@@ -200,6 +206,12 @@ func setActive() {
 		iup.GetHandle("VboxQuality").SetAttribute("ACTIVE", "NO")
 	}
 
+	if (opts.Format == "webp" || opts.Format == "avif" || opts.Format == "jxl") && !opts.NoConvert {
+		iup.GetHandle("VboxEffort").SetAttribute("ACTIVE", "YES")
+	} else {
+		iup.GetHandle("VboxEffort").SetAttribute("ACTIVE", "NO")
+	}
+
 	if opts.Width != 0 && opts.Height != 0 && !opts.NoConvert {
 		iup.GetHandle("Fit").SetAttribute("ACTIVE", "YES")
 	} else {
@@ -207,6 +219,32 @@ func setActive() {
 	}
 }
 
+func setEffort(format string) {
+	val := iup.GetHandle("Effort")
+
+	var name string
+
+	switch format {
+	case "webp":
+		val.SetAttributes("MIN=0, MAX=6, SHOWTICKS=7, VALUE=4")
+		val.SetAttribute("TIP", "WEBP method, higher is better/slower (0-6, default 4)")
+		name = "Method"
+	case "avif":
+		val.SetAttributes("MIN=0, MAX=10, SHOWTICKS=11, VALUE=10")
+		val.SetAttribute("TIP", "AVIF speed, higher is faster/worse (0-10, default 10)")
+		name = "Speed"
+	case "jxl":
+		val.SetAttributes("MIN=1, MAX=10, SHOWTICKS=10, VALUE=7")
+		val.SetAttribute("TIP", "JXL effort, higher is better/slower (1-10, default 7)")
+		name = "Effort"
+	default:
+		return
+	}
+
+	val.SetAttribute("EFFORTNAME", name)
+	iup.GetHandle("LabelEffort").SetAttribute("TITLE", fmt.Sprintf("%s: %d", name, val.GetInt("VALUE")))
+}
+
 func layout() iup.Ihandle {
 	return iup.Vbox(
 		iup.Hbox(
@@ -409,6 +447,7 @@ func tabs() iup.Ihandle {
 				"7":        "JXL",
 			}).SetHandle("Format").
 				SetCallback("VALUECHANGED_CB", iup.ValueChangedFunc(func(ih iup.Ihandle) int {
+					setEffort(strings.ToLower(ih.GetAttribute("VALUESTRING")))
 					setActive()
 					previewPost()
 
@@ -494,6 +533,25 @@ func tabs() iup.Ihandle {
 					return iup.DEFAULT
 				})),
 		).SetHandle("VboxQuality"),
+		iup.Vbox(
+			iup.Label("").SetHandle("LabelEffort"),
+			iup.Val("").SetAttributes(`MIN=0, MAX=10, VALUE=0, SHOWTICKS=11`).SetHandle("Effort").
+				SetAttribute("TIP", "Encoder speed/effort (WEBP, AVIF, JXL)").
+				SetCallback("VALUECHANGED_CB", iup.ValueChangedFunc(func(ih iup.Ihandle) int {
+					iup.GetHandle("LabelEffort").SetAttribute("TITLE", fmt.Sprintf("%s: %d", ih.GetAttribute("EFFORTNAME"), ih.GetInt("VALUE")))
+					ih.SetAttribute("MYVALUE", ih.GetInt("VALUE"))
+
+					return iup.DEFAULT
+				})).
+				SetCallback("KILLFOCUS_CB", iup.KillFocusFunc(func(ih iup.Ihandle) int {
+					if ih.GetAttribute("MYVALUE") != "" {
+						previewPost()
+					}
+					ih.SetAttribute("MYVALUE", "")
+
+					return iup.DEFAULT
+				})),
+		).SetHandle("VboxEffort"),
 		iup.Vbox(
 			iup.Toggle(" Grayscale").SetHandle("Grayscale").
 				SetAttributes(`TIP="Convert images to grayscale (monochromatic)"`).
diff --git a/cmd/cbconvert/main.go b/cmd/cbconvert/main.go
index a6cde7c..d438471 100644
--- a/cmd/cbconvert/main.go
+++ b/cmd/cbconvert/main.go
@@ -167,6 +167,7 @@ func parseFlags() (cbconvert.Options, []string) {
 	convert.StringVar(&opts.Format, "format", "jpeg", "Image format, valid values are jpeg, png, tiff, bmp, webp, avif, jxl")
 	convert.StringVar(&opts.Archive, "archive", "zip", "Archive format, valid values are zip, tar")
 	convert.IntVar(&opts.Quality, "quality", 75, "Image quality")
+	convert.IntVar(&opts.Effort, "effort", -1, "Encoder speed/effort, format-specific (webp method 0-6, avif speed 0-10, jxl effort 1-10), -1 uses the format default")
 	convert.IntVar(&opts.Filter, "filter", 2, "0=NearestNeighbor, 1=Box, 2=Linear, 3=MitchellNetravali, 4=CatmullRom, 6=Gaussian, 7=Lanczos")
 	convert.BoolVar(&opts.NoCover, "no-cover", false, "Do not convert the cover image")
 	convert.BoolVar(&opts.NoRGB, "no-rgb", false, "Do not convert images that have RGB colorspace")
@@ -188,6 +189,7 @@ func parseFlags() (cbconvert.Options, []string) {
 	cover.BoolVar(&opts.Fit, "fit", false, "Best fit for required width and height")
 	cover.StringVar(&opts.Format, "format", "jpeg", "Image format, valid values are jpeg, png, tiff, bmp, webp, avif")
 	cover.IntVar(&opts.Quality, "quality", 75, "Image quality")
+	cover.IntVar(&opts.Effort, "effort", -1, "Encoder speed/effort, format-specific (webp method 0-6, avif speed 0-10, jxl effort 1-10), -1 uses the format default")
 	cover.IntVar(&opts.Filter, "filter", 2, "0=NearestNeighbor, 1=Box, 2=Linear, 3=MitchellNetravali, 4=CatmullRom, 6=Gaussian, 7=Lanczos")
 	cover.StringVar(&opts.OutDir, "outdir", ".", "Output directory")
 	cover.IntVar(&opts.Size, "size", 0, "Process only files larger than size (in MB)")
@@ -218,7 +220,7 @@ func parseFlags() (cbconvert.Options, []string) {
 		fmt.Fprintf(os.Stderr, "Usage: %s <command> [<flags>] [file1 dir1 ... fileOrDirN]\n\n", filepath.Base(os.Args[0]))
 		fmt.Fprintf(os.Stderr, "\nCommands:\n")
 		fmt.Fprintf(os.Stderr, "\n  convert\n    \tConvert archive or document\n\n")
-		order := []string{"width", "height", "fit", "format", "archive", "quality", "filter", "no-cover", "no-rgb",
+		order := []string{"width", "height", "fit", "format", "archive", "quality", "effort", "filter", "no-cover", "no-rgb",
 			"no-nonimage", "no-convert", "grayscale", "rotate", "brightness", "contrast", "suffix", "outdir", "size", "recursive", "quiet"}
 		for _, name := range order {
 			f := convert.Lookup(name)
@@ -226,7 +228,7 @@ func parseFlags() (cbconvert.Options, []string) {
 			fmt.Fprintf(os.Stderr, "%v (default %q)\n", f.Usage, f.DefValue)
 		}
 		fmt.Fprintf(os.Stderr, "\n  cover\n    \tExtract cover\n\n")
-		order = []string{"width", "height", "fit", "format", "quality", "filter", "outdir", "size", "recursive", "quiet"}
+		order = []string{"width", "height", "fit", "format", "quality", "effort", "filter", "outdir", "size", "recursive", "quiet"}
 		for _, name := range order {
 			f := cover.Lookup(name)
 			fmt.Fprintf(os.Stderr, "    --%s\n    \t", f.Name)
@@ -297,7 +299,7 @@ func parseFlags() (cbconvert.Options, []string) {
 	return opts, args
 }
 
-// piped checks if we have a piped stdin.
+// piped checks if we have piped stdin.
 func piped() bool {
 	f, err := os.Stdin.Stat()
 	if err != nil {
@@ -311,7 +313,7 @@ func piped() bool {
 	return true
 }
 
-// lines returns slice of lines from reader.
+// lines returns slice of lines from the reader.
 func lines(r io.Reader) []string {
 	data := make([]string, 0)
 	scanner := bufio.NewScanner(r)