diff --git a/td/generate/tl_json_converter.cpp b/td/generate/tl_json_converter.cpp
index 8f4ee721c..673306aed 100644
--- a/td/generate/tl_json_converter.cpp
+++ b/td/generate/tl_json_converter.cpp
@@ -136,7 +136,7 @@ void gen_tl_constructor_from_string(StringBuilder &sb, Slice name, const Vec &ve
     return;
   }
   sb << " {\n";
-  sb << "  static const std::unordered_map<Slice, int32, SliceHash> m = {\n";
+  sb << "  static const FlatHashMap<Slice, int32, SliceHash> m = {\n";
 
   bool is_first = true;
   for (auto &p : vec) {
@@ -216,10 +216,10 @@ void gen_json_converter_file(const tl::simple::Schema &schema, const std::string
 
     sb << "#include \"td/utils/base64.h\"\n";
     sb << "#include \"td/utils/common.h\"\n";
+    sb << "#include \"td/utils/FlatHashMap.h\"\n";
     sb << "#include \"td/utils/Slice.h\"\n\n";
 
-    sb << "#include <functional>\n";
-    sb << "#include <unordered_map>\n\n";
+    sb << "#include <functional>\n\n";
   }
   sb << "namespace td {\n";
   sb << "namespace td_api {\n";
diff --git a/td/mtproto/RawConnection.cpp b/td/mtproto/RawConnection.cpp
index 00c1e1f8d..a663e6af3 100644
--- a/td/mtproto/RawConnection.cpp
+++ b/td/mtproto/RawConnection.cpp
@@ -15,6 +15,7 @@
 #include "td/net/DarwinHttp.h"
 #endif
 
+#include "td/utils/FlatHashMap.h"
 #include "td/utils/format.h"
 #include "td/utils/logging.h"
 #include "td/utils/misc.h"
@@ -26,7 +27,6 @@
 #include "td/utils/StorerBase.h"
 
 #include <memory>
-#include <unordered_map>
 #include <utility>
 
 namespace td {
@@ -67,6 +67,7 @@ class RawConnectionDefault final : public RawConnection {
 
     bool use_quick_ack = false;
     if (quick_ack_token != 0 && transport_->support_quick_ack()) {
+      CHECK(info.message_ack & (1u << 31));
       auto tmp = quick_ack_to_token_.emplace(info.message_ack, quick_ack_token);
       if (tmp.second) {
         use_quick_ack = true;
@@ -130,7 +131,7 @@ class RawConnectionDefault final : public RawConnection {
   PublicFields extra_;
   BufferedFd<SocketFd> socket_fd_;
   unique_ptr<IStreamTransport> transport_;
-  std::unordered_map<uint32, uint64> quick_ack_to_token_;
+  FlatHashMap<uint32, uint64> quick_ack_to_token_;
   bool has_error_{false};
 
   unique_ptr<StatsCallback> stats_callback_;
@@ -218,11 +219,15 @@ class RawConnectionDefault final : public RawConnection {
   }
 
   Status on_quick_ack(uint32 quick_ack, Callback &callback) {
+    if ((quick_ack & (1u << 31)) == 0) {
+      LOG(ERROR) << "Receive invalid quick_ack " << quick_ack;
+      return Status::OK();
+    }
+
     auto it = quick_ack_to_token_.find(quick_ack);
     if (it == quick_ack_to_token_.end()) {
-      LOG(WARNING) << Status::Error(PSLICE() << "Unknown quick_ack " << quick_ack);
+      LOG(WARNING) << "Receive unknown quick_ack " << quick_ack;
       return Status::OK();
-      // TODO: return Status::Error(PSLICE() << "Unknown quick_ack " << quick_ack);
     }
     auto token = it->second;
     quick_ack_to_token_.erase(it);
diff --git a/td/mtproto/SessionConnection.cpp b/td/mtproto/SessionConnection.cpp
index 58380aa64..05f3dfa0c 100644
--- a/td/mtproto/SessionConnection.cpp
+++ b/td/mtproto/SessionConnection.cpp
@@ -437,13 +437,13 @@ Status SessionConnection::on_packet(const MsgInfo &info, const mtproto_api::msgs
   if (it == service_queries_.end()) {
     return Status::Error("Unknown msgs_state_info");
   }
-  SCOPE_EXIT {
-    service_queries_.erase(it);
-  };
-  if (it->second.type != ServiceQuery::GetStateInfo) {
-    return Status::Error("Got msg_state_info in response not to GetStateInfo");
+  auto query = std::move(it->second);
+  service_queries_.erase(it);
+
+  if (query.type != ServiceQuery::GetStateInfo) {
+    return Status::Error("Receive msg_state_info in response not to GetStateInfo");
   }
-  return on_msgs_state_info(it->second.message_ids, msgs_state_info.info_);
+  return on_msgs_state_info(query.message_ids, msgs_state_info.info_);
 }
 
 Status SessionConnection::on_packet(const MsgInfo &info, const mtproto_api::msgs_all_info &msgs_all_info) {
@@ -573,8 +573,9 @@ void SessionConnection::on_message_failed(uint64 id, Status status) {
 
   auto cit = container_to_service_msg_.find(id);
   if (cit != container_to_service_msg_.end()) {
-    for (auto nid : cit->second) {
-      on_message_failed_inner(nid);
+    auto message_ids = cit->second;
+    for (auto message_id : message_ids) {
+      on_message_failed_inner(message_id);
     }
   } else {
     on_message_failed_inner(id);
@@ -586,21 +587,25 @@ void SessionConnection::on_message_failed_inner(uint64 id) {
   if (it == service_queries_.end()) {
     return;
   }
-  switch (it->second.type) {
+  auto query = std::move(it->second);
+  service_queries_.erase(it);
+
+  switch (query.type) {
     case ServiceQuery::ResendAnswer: {
-      for (auto message_id : it->second.message_ids) {
+      for (auto message_id : query.message_ids) {
         resend_answer(message_id);
       }
       break;
     }
     case ServiceQuery::GetStateInfo: {
-      for (auto message_id : it->second.message_ids) {
+      for (auto message_id : query.message_ids) {
         get_state_info(message_id);
       }
       break;
     }
+    default:
+      UNREACHABLE();
   }
-  service_queries_.erase(id);
 }
 
 bool SessionConnection::must_flush_packet() {
@@ -968,11 +973,10 @@ void SessionConnection::flush_packet() {
   }
 
   if (resend_answer_id) {
-    service_queries_.insert({resend_answer_id, ServiceQuery{ServiceQuery::ResendAnswer, std::move(to_resend_answer)}});
+    service_queries_.emplace(resend_answer_id, ServiceQuery{ServiceQuery::ResendAnswer, std::move(to_resend_answer)});
   }
   if (get_state_info_id) {
-    service_queries_.insert(
-        {get_state_info_id, ServiceQuery{ServiceQuery::GetStateInfo, std::move(to_get_state_info)}});
+    service_queries_.emplace(get_state_info_id, ServiceQuery{ServiceQuery::GetStateInfo, std::move(to_get_state_info)});
   }
   if (ping_id != 0) {
     last_ping_container_id_ = container_id;
diff --git a/td/mtproto/SessionConnection.h b/td/mtproto/SessionConnection.h
index 6ca7524bf..8446fd1c0 100644
--- a/td/mtproto/SessionConnection.h
+++ b/td/mtproto/SessionConnection.h
@@ -11,6 +11,7 @@
 #include "td/mtproto/RawConnection.h"
 
 #include "td/utils/buffer.h"
+#include "td/utils/FlatHashMap.h"
 #include "td/utils/format.h"
 #include "td/utils/logging.h"
 #include "td/utils/Named.h"
@@ -22,7 +23,6 @@
 #include "td/utils/StringBuilder.h"
 #include "td/utils/tl_parsers.h"
 
-#include <unordered_map>
 #include <utility>
 
 namespace td {
@@ -176,10 +176,10 @@ class SessionConnection final
   vector<int64> to_resend_answer_;
   vector<int64> to_cancel_answer_;
   vector<int64> to_get_state_info_;
-  std::unordered_map<uint64, ServiceQuery> service_queries_;
+  FlatHashMap<uint64, ServiceQuery> service_queries_;
 
   // nobody cleans up this map. But it should be really small.
-  std::unordered_map<uint64, vector<uint64>> container_to_service_msg_;
+  FlatHashMap<uint64, vector<uint64>> container_to_service_msg_;
 
   double last_read_at_ = 0;
   double last_ping_at_ = 0;
diff --git a/td/telegram/ContactsManager.cpp b/td/telegram/ContactsManager.cpp
index 4cf82168d..664db1645 100644
--- a/td/telegram/ContactsManager.cpp
+++ b/td/telegram/ContactsManager.cpp
@@ -3441,13 +3441,15 @@ void ContactsManager::on_channel_participant_cache_timeout(ChannelId channel_id)
 
   auto &participants = channel_participants_it->second.participants_;
   auto min_access_date = G()->unix_time() - CHANNEL_PARTICIPANT_CACHE_TIME;
-  for (auto it = participants.begin(); it != participants.end();) {
+  vector<DialogId> to_delete_dialog_ids;
+  for (auto it = participants.begin(); it != participants.end(); ++it) {
     if (it->second.last_access_date_ < min_access_date) {
-      it = participants.erase(it);
-    } else {
-      ++it;
+      to_delete_dialog_ids.push_back(it->first);
     }
   }
+  for (auto dialog_id : to_delete_dialog_ids) {
+    participants.erase(dialog_id);
+  }
 
   if (participants.empty()) {
     channel_participants_.erase(channel_participants_it);
diff --git a/td/telegram/ContactsManager.h b/td/telegram/ContactsManager.h
index 35c31f980..4c1cb5c89 100644
--- a/td/telegram/ContactsManager.h
+++ b/td/telegram/ContactsManager.h
@@ -48,7 +48,6 @@
 
 #include <functional>
 #include <memory>
-#include <unordered_map>
 #include <unordered_set>
 #include <utility>
 
@@ -1742,7 +1741,7 @@ class ContactsManager final : public Actor {
     int32 last_access_date_ = 0;
   };
   struct ChannelParticipants {
-    std::unordered_map<DialogId, ChannelParticipantInfo, DialogIdHash> participants_;
+    FlatHashMap<DialogId, ChannelParticipantInfo, DialogIdHash> participants_;
   };
   FlatHashMap<ChannelId, ChannelParticipants, ChannelIdHash> channel_participants_;