diff --git a/src/liballoc/string.rs b/src/liballoc/string.rs
index 8d009101ce7da..0b25d911a299c 100644
--- a/src/liballoc/string.rs
+++ b/src/liballoc/string.rs
@@ -618,6 +618,8 @@ impl String {
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn from_utf16(v: &[u16]) -> Result<String, FromUtf16Error> {
+        // This isn't done via collect::<Result<_, _>>() for performance reasons.
+        // FIXME: the function can be simplified again when #48994 is closed.
         let mut ret = String::with_capacity(v.len());
         for c in decode_utf16(v.iter().cloned()) {
             if let Ok(c) = c {
diff --git a/src/libserialize/collection_impls.rs b/src/libserialize/collection_impls.rs
index 3e028d755c6d4..cbd642dd6ad91 100644
--- a/src/libserialize/collection_impls.rs
+++ b/src/libserialize/collection_impls.rs
@@ -86,7 +86,7 @@ impl<T: Encodable> Encodable for VecDeque<T> {
 impl<T:Decodable> Decodable for VecDeque<T> {
     fn decode<D: Decoder>(d: &mut D) -> Result<VecDeque<T>, D::Error> {
         d.read_seq(|d, len| {
-            let mut deque: VecDeque<T> = VecDeque::new();
+            let mut deque: VecDeque<T> = VecDeque::with_capacity(len);
             for i in 0..len {
                 deque.push_back(d.read_seq_elt(i, |d| Decodable::decode(d))?);
             }