Disable some reductions at low levels

src/reduction/mod.rs

@@ -1,6 +1,7 @@
 use crate::evaluate::Evaluator;
 use crate::png::PngImage;
 use crate::Deadline;
+use crate::Deflaters;
 use crate::Options;
 use std::sync::Arc;
 
@@ -22,6 +23,13 @@ pub(crate) fn perform_reductions(
     let mut reduction_occurred = false;
     let mut evaluation_added = false;
 
+    // At low compression levels, skip some transformations which are less likely to be effective
+    // This currently affects optimization presets 0-2
+    let cheap = match opts.deflate {
+        Deflaters::Libdeflater { compression } => compression < 12 && opts.fast_evaluation,
+        _ => false,
+    };
+
     // Interlacing must be processed first in order to evaluate the rest correctly
     if let Some(interlacing) = opts.interlace {
         if let Some(reduced) = png.change_interlacing(interlacing) {
@@ -97,7 +105,7 @@ pub(crate) fn perform_reductions(
 
     // Attempt to convert from indexed to channels
     // This may give a better result due to dropping the PLTE chunk
-    if opts.color_type_reduction && !deadline.passed() {
+    if !cheap && opts.color_type_reduction && !deadline.passed() {
         if let Some(reduced) = indexed_to_channels(&png, opts.grayscale_reduction) {
             // This result should not be passed on to subsequent reductions
             eval.try_image(Arc::new(reduced));
@@ -125,7 +133,7 @@ pub(crate) fn perform_reductions(
     }
 
     // Attempt to sort the palette using an alternative method
-    if opts.palette_reduction && !deadline.passed() {
+    if !cheap && opts.palette_reduction && !deadline.passed() {
         if let Some(reduced) = sorted_palette_battiato(&png) {
             eval.try_image(Arc::new(reduced));
             evaluation_added = true;