diff --git a/compiler/solve/src/solve.rs b/compiler/solve/src/solve.rs
index 51d1035b80..5c1263218d 100644
--- a/compiler/solve/src/solve.rs
+++ b/compiler/solve/src/solve.rs
@@ -1175,23 +1175,25 @@ fn check_for_infinite_type(
                     },
                 );
 
-                let mut new_tags = Vec::with_capacity(tags.len());
+                let new_variable_slices = SubsSlice::reserve_variable_slices(subs, tags.len());
 
-                for (name_index, slice_index) in tags.iter_all() {
+                let it = new_variable_slices.indices().zip(tags.iter_all());
+                for (variable_slice_index, (_, slice_index)) in it {
                     let slice = subs[slice_index];
 
-                    let mut new_vars = Vec::new();
-                    for var_index in slice {
+                    let new_variables = VariableSubsSlice::reserve_into_subs(subs, slice.len());
+                    for (target_index, var_index) in new_variables.indices().zip(slice) {
                         let var = subs[var_index];
-                        new_vars.push(subs.explicit_substitute(recursive, rec_var, var));
+                        subs.variables[target_index] =
+                            subs.explicit_substitute(recursive, rec_var, var);
                     }
 
-                    new_tags.push((subs[name_index].clone(), new_vars));
+                    subs.variable_slices[variable_slice_index] = new_variables;
                 }
 
                 let new_ext_var = subs.explicit_substitute(recursive, rec_var, ext_var);
 
-                let new_tags = UnionTags::insert_into_subs(subs, new_tags);
+                let new_tags = UnionTags::from_slices(tags.tag_names(), new_variable_slices);
 
                 let flat_type = FlatType::RecursiveTagUnion(rec_var, new_tags, new_ext_var);
 
diff --git a/compiler/types/src/subs.rs b/compiler/types/src/subs.rs
index f4d95fa0c3..8569d0771e 100644
--- a/compiler/types/src/subs.rs
+++ b/compiler/types/src/subs.rs
@@ -1983,9 +1983,11 @@ impl RecordFields {
             SubsIndex<RecordField<()>>,
         ),
     > {
-        let range1 = self.field_names_start..self.field_names_start + self.length as u32;
-        let range2 = self.variables_start..self.variables_start + self.length as u32;
-        let range3 = self.field_types_start..self.field_types_start + self.length as u32;
+        let helper = |start| start..(start + self.length as u32);
+
+        let range1 = helper(self.field_names_start);
+        let range2 = helper(self.variables_start);
+        let range3 = helper(self.field_types_start);
 
         let it = range1
             .into_iter()
@@ -3247,8 +3249,7 @@ fn deep_copy_var_to_help<'a>(
 
                     let new_tag_names = {
                         let tag_names = tags.tag_names();
-                        let slice = &source.tag_names[tag_names.start as usize..]
-                            [..tag_names.length as usize];
+                        let slice = &source.tag_names[tag_names.indices()];
 
                         let start = target.tag_names.len() as u32;
                         let length = tag_names.len() as u16;
@@ -3305,8 +3306,7 @@ fn deep_copy_var_to_help<'a>(
 
                     let new_tag_names = {
                         let tag_names = tags.tag_names();
-                        let slice = &source.tag_names[tag_names.start as usize..]
-                            [..tag_names.length as usize];
+                        let slice = &source.tag_names[tag_names.indices()];
 
                         let start = target.tag_names.len() as u32;
                         let length = tag_names.len() as u16;