diff --git a/power/VcdReader.cc b/power/VcdReader.cc
index 6655686b..24c539dd 100644
--- a/power/VcdReader.cc
+++ b/power/VcdReader.cc
@@ -25,7 +25,7 @@
 #include "VcdReader.hh"
 
 #include <inttypes.h>
-#include <set>
+#include <unordered_map>
 
 #include "VcdParse.hh"
 #include "Debug.hh"
@@ -43,7 +43,7 @@ using std::abs;
 using std::min;
 using std::to_string;
 using std::vector;
-using std::map;
+using std::unordered_map;
 
 // Transition count and high time for duty cycle for a group of pins
 // for one bit of vcd ID.
@@ -116,7 +116,7 @@ VcdCount::highTime(VcdTime time_max) const
 // VcdCount[bit]
 typedef vector<VcdCount> VcdCounts;
 // ID -> VcdCount[bit]
-typedef map<string, VcdCounts> VcdIdCountsMap;
+typedef unordered_map<string, VcdCounts> VcdIdCountsMap;
 
 class VcdCountReader : public VcdReader
 {
@@ -290,7 +290,7 @@ VcdCountReader::varAppendValue(const string &id,
                                VcdTime time,
                                char value)
 {
-  auto itr = vcd_count_map_.find(id);
+  const auto &itr = vcd_count_map_.find(id);
   if (itr != vcd_count_map_.end()) {
     VcdCounts &vcd_counts = itr->second;
     if (debug_->check("read_vcd_activities", 3)) {
@@ -316,7 +316,7 @@ VcdCountReader::varAppendBusValue(const string &id,
                                   VcdTime time,
                                   int64_t bus_value)
 {
-  auto itr = vcd_count_map_.find(id);
+  const auto &itr = vcd_count_map_.find(id);
   if (itr != vcd_count_map_.end()) {
     VcdCounts &vcd_counts = itr->second;
     for (size_t bit_idx = 0; bit_idx < vcd_counts.size(); bit_idx++) {