diff --git a/tracing-core/src/span.rs b/tracing-core/src/span.rs
index 1cb228d38d..7deb7c12df 100644
--- a/tracing-core/src/span.rs
+++ b/tracing-core/src/span.rs
@@ -195,6 +195,12 @@ impl<'a> Attributes<'a> {
     pub fn is_empty(&self) -> bool {
         self.values.is_empty()
     }
+
+    /// Returns the number of fields in this `Attributes`.
+    #[inline]
+    pub fn len(&self) -> usize {
+        self.values.field_set().len()
+    }
 }
 
 // ===== impl Record =====
diff --git a/tracing-opentelemetry/src/subscriber.rs b/tracing-opentelemetry/src/subscriber.rs
index cb2b7aa4ce..3ae4b720ec 100644
--- a/tracing-opentelemetry/src/subscriber.rs
+++ b/tracing-opentelemetry/src/subscriber.rs
@@ -171,17 +171,20 @@ impl<'a> field::Visit for SpanEventVisitor<'a> {
 
 struct SpanAttributeVisitor<'a>(&'a mut otel::SpanBuilder);
 
+impl<'a> SpanAttributeVisitor<'a> {
+    fn record(&mut self, attribute: KeyValue) {
+        debug_assert!(self.0.attributes.is_some());
+        self.0.attributes.as_mut().map(|v| v.push(attribute));
+    }
+}
+
 impl<'a> field::Visit for SpanAttributeVisitor<'a> {
     /// Set attributes on the underlying OpenTelemetry [`Span`] from `bool` values.
     ///
     /// [`Span`]: opentelemetry::trace::Span
     fn record_bool(&mut self, field: &field::Field, value: bool) {
         let attribute = KeyValue::new(field.name(), value);
-        if let Some(attributes) = &mut self.0.attributes {
-            attributes.push(attribute);
-        } else {
-            self.0.attributes = Some(vec![attribute]);
-        }
+        self.record(attribute);
     }
 
     /// Set attributes on the underlying OpenTelemetry [`Span`] from `i64` values.
@@ -189,11 +192,7 @@ impl<'a> field::Visit for SpanAttributeVisitor<'a> {
     /// [`Span`]: opentelemetry::trace::Span
     fn record_i64(&mut self, field: &field::Field, value: i64) {
         let attribute = KeyValue::new(field.name(), value);
-        if let Some(attributes) = &mut self.0.attributes {
-            attributes.push(attribute);
-        } else {
-            self.0.attributes = Some(vec![attribute]);
-        }
+        self.record(attribute);
     }
 
     /// Set attributes on the underlying OpenTelemetry [`Span`] from `&str` values.
@@ -206,11 +205,7 @@ impl<'a> field::Visit for SpanAttributeVisitor<'a> {
             self.0.span_kind = str_to_span_kind(value);
         } else {
             let attribute = KeyValue::new(field.name(), value.to_string());
-            if let Some(attributes) = &mut self.0.attributes {
-                attributes.push(attribute);
-            } else {
-                self.0.attributes = Some(vec![attribute]);
-            }
+            self.record(attribute);
         }
     }
 
@@ -225,11 +220,7 @@ impl<'a> field::Visit for SpanAttributeVisitor<'a> {
             self.0.span_kind = str_to_span_kind(&format!("{:?}", value));
         } else {
             let attribute = Key::new(field.name()).string(format!("{:?}", value));
-            if let Some(attributes) = &mut self.0.attributes {
-                attributes.push(attribute);
-            } else {
-                self.0.attributes = Some(vec![attribute]);
-            }
+            self.record(attribute);
         }
     }
 }
@@ -418,6 +409,7 @@ where
             builder.trace_id = Some(self.tracer.new_trace_id());
         }
 
+        builder.attributes = Some(Vec::with_capacity(attrs.len()));
         attrs.record(&mut SpanAttributeVisitor(&mut builder));
         extensions.insert(builder);
     }