compared via this jq script i wrote

def prettify: {
  "attrset": {
    "lookups": .nrLookups,
    "merges": .nrOpUpdates,
    "mergeCopies": .nrOpUpdateValuesCopied
  },
  "list": {
    "concats": .list.concats,
  },
  "parser": {
    expressions: .nrExprs
  },
  "memory": {
    "envs": .envs.bytes,
    "list": .list.bytes,
    "sets": .sets.bytes,
    "symbols": .symbols.bytes,
    "values": .values.bytes,
    "total": .envs.bytes + .list.bytes + .sets.bytes + .symbols.bytes + .values.bytes
  },
  "speed": {
    "primops": .nrPrimOpCalls,
    "functionCalls": .nrFunctionCalls,
    "thunksMade": .nrThunks,
    "thunksAvoided": .nrAvoided,
  }
};

def display_number:
if . == 0 then
  null
else
  if . > 0 then
    "+" + (. | tostring) + "%"
  else
    (. | tostring) + "%"
  end
end;

def percentage(places):
  # Round to the nearest n decimal places (after the decimal place)
  . * pow(10; places + 2) | round / pow(10; places) | tostring;

def display_percentage:
  if . == 0 then
    null
  else
    if . > 0 then
      "+" + (. | percentage(2)) + "%"
    else
      (. | percentage(2)) + "%"
    end
  end;


[(.[] | prettify)] as [$stats_before, $stats_after] |
reduce ($stats_before | paths(numbers)) as $path (
  $stats_after;
    setpath(
      $path;
      getpath($path) as $after |
      ($stats_before | getpath($path)) as $before |
      if $before == 0 then
        false
      else
        (($after - $before) / $before) | display_percentage
      end
    )
)

  nodelike = lib.mkOptionType {
    name = "nodelike";
    description = "Something that acts like a node, with attributes, a value, and children.";
    descriptionClass = "noun";
    check = val: true;
    merge = loc: defs: if (builtins.length defs == 1)
      then (builtins.head defs).value
      else (builtins.foldl' (acc: def: let
        acc-def = { value = acc.val; file = acc.file; };
        fail = throw "The option `${lib.options.showOption loc}` has conflicting option types:${lib.options.showDefs [ acc-def def ]}";
      in if (acc.type == "list")
        then if (builtins.typeOf def.value) == "list"
          then { type = "list"; file = def.file; val = (lib.types.listOf nodelike).merge loc [ acc-def def ]; }
          else if (builtins.typeOf def.value) == "set"
            then {
              type = "list"; file = def.file;
              val = (lib.types.listOf nodelike).merge loc [
                acc-def
                { value = [ def.value ]; file = def.file; }
              ];
            } else fail
        else if (acc.type == "set")
          then if (builtins.typeOf def.value) == "set"
            then { type = "set"; file = def.file; val = (lib.types.attrsOf nodelike).merge loc [ acc-def def ]; }
            else if (builtins.typeOf def.value) == "list"
              then {
                type = "list"; file = def.file;
                val = (lib.types.listOf nodelike).merge loc [
                  { value = [ acc.val ]; file = acc.file; }
                  def
                ];
              } else fail
          else if (builtins.typeOf def.value) == acc.type
            then { type = acc.type; file = def.file; val = lib.options.mergeEqualOption loc [ acc-def def ]; }
            else fail
      ) (let h = builtins.head defs; in { type = builtins.typeOf h; file = h.file; val = h.value; }) (builtins.tail defs)).val;
  };

There are some more problems in your code.

h = builtins.head defs; # -> definition :: { file, value }
type = builtins.typeOf h; # -> attrs, always