diff --git a/components/style/properties/declaration_block.rs b/components/style/properties/declaration_block.rs
index c332b4b07da..efbccd9f991 100644
--- a/components/style/properties/declaration_block.rs
+++ b/components/style/properties/declaration_block.rs
@@ -14,6 +14,7 @@ use parser::{ParserContext, log_css_error};
 use properties::animated_properties::AnimationValue;
 use selectors::parser::SelectorParseError;
 use shared_lock::Locked;
+use smallvec::SmallVec;
 use std::fmt;
 use std::slice::Iter;
 use style_traits::{PARSING_MODE_DEFAULT, ToCss, ParseError, ParsingMode, StyleParseError};
@@ -605,24 +606,31 @@ impl ToCss for PropertyDeclarationBlock {
             // Step 3.3
             let shorthands = declaration.shorthands();
             if !shorthands.is_empty() {
-                // Step 3.3.1
-                //
-                // FIXME(emilio): All this looks terribly inefficient...
-                let mut longhands = self.declarations.iter()
-                    .filter(|d| !already_serialized.contains(d.0.id()))
-                    .collect::<Vec<_>>();
+                // Step 3.3.1 is done by checking already_serialized while
+                // iterating below.
 
                 // Step 3.3.2
                 for &shorthand in shorthands {
                     let properties = shorthand.longhands();
 
                     // Substep 2 & 3
-                    let mut current_longhands = Vec::new();
+                    let mut current_longhands = SmallVec::<[_; 10]>::new();
                     let mut important_count = 0;
                     let mut found_system = None;
 
-                    if shorthand == ShorthandId::Font && longhands.iter().any(|&&(ref l, _)| l.get_system().is_some()) {
-                        for &&(ref longhand, longhand_importance) in longhands.iter() {
+                    let is_system_font =
+                        shorthand == ShorthandId::Font &&
+                        self.declarations.iter().any(|&(ref l, _)| {
+                            !already_serialized.contains(l.id()) &&
+                            l.get_system().is_some()
+                        });
+
+                    if is_system_font {
+                        for &(ref longhand, longhand_importance) in &self.declarations {
+                            if already_serialized.contains(longhand.id()) {
+                                continue;
+                            }
+
                             if longhand.get_system().is_some() || longhand.is_default_line_height() {
                                 current_longhands.push(longhand);
                                 if found_system.is_none() {
@@ -634,7 +642,11 @@ impl ToCss for PropertyDeclarationBlock {
                             }
                         }
                     } else {
-                        for &&(ref longhand, longhand_importance) in longhands.iter() {
+                        for &(ref longhand, longhand_importance) in &self.declarations {
+                            if already_serialized.contains(longhand.id()) {
+                                continue;
+                            }
+
                             if longhand.id().is_longhand_of(shorthand) {
                                 current_longhands.push(longhand);
                                 if longhand_importance.important() {
@@ -728,11 +740,6 @@ impl ToCss for PropertyDeclarationBlock {
                     for current_longhand in &current_longhands {
                         // Substep 9
                         already_serialized.insert(current_longhand.id());
-                        let index_to_remove = longhands.iter().position(|l| l.0 == **current_longhand);
-                        if let Some(index) = index_to_remove {
-                            // Substep 10
-                            longhands.remove(index);
-                        }
                     }
                 }
             }