diff --git a/app/models/rooms/direct.rb b/app/models/rooms/direct.rb
index ba7856b..18430d5 100644
--- a/app/models/rooms/direct.rb
+++ b/app/models/rooms/direct.rb
@@ -7,12 +7,15 @@ class Rooms::Direct < Room
     end
 
     private
-      # FIXME: Find a more performant algorithm that won't be a problem on accounts with 10K+ direct rooms,
-      # which could be to store the membership id list as a hash on the room, and use that for lookup.
+      # Among the first user's rooms, the one whose members are exactly these users: as many
+      # memberships as users, and all of them theirs.
       def find_for(users)
-        all.joins(:users).detect do |room|
-          Set.new(room.user_ids) == Set.new(users.pluck(:id))
-        end
+        user_ids = users.pluck(:id).uniq
+
+        where(id: Membership.where(user_id: user_ids.first).select(:room_id))
+          .joins(:memberships).group(:id)
+          .having("COUNT(*) = :size AND COUNT(CASE WHEN memberships.user_id IN (:user_ids) THEN 1 END) = :size", size: user_ids.size, user_ids: user_ids)
+          .first
       end
   end
 
diff --git a/test/models/rooms/direct_test.rb b/test/models/rooms/direct_test.rb
index ab74efa..b3f2722 100644
--- a/test/models/rooms/direct_test.rb
+++ b/test/models/rooms/direct_test.rb
@@ -18,4 +18,33 @@ class Rooms::DirectTest < ActiveSupport::TestCase
     room = Rooms::Direct.find_or_create_for([ users(:david), users(:kevin) ])
     assert room.memberships.all? { |m| m.involved_in_everything? }
   end
+
+  test "find the room with exactly the same users" do
+    Current.user = users(:david)
+    group = Rooms::Direct.find_or_create_for([ users(:david), users(:kevin), users(:jason) ])
+
+    assert_equal rooms(:david_and_kevin), Rooms::Direct.find_or_create_for([ users(:kevin), users(:david) ])
+    assert_equal group, Rooms::Direct.find_or_create_for(User.where(id: [ users(:jason), users(:kevin), users(:david) ]))
+    assert_equal [ users(:david), users(:jz) ].to_set, Rooms::Direct.find_or_create_for([ users(:david), users(:jz) ]).users.to_set
+    assert_equal [ users(:jason), users(:kevin) ].to_set, Rooms::Direct.find_or_create_for([ users(:jason), users(:kevin) ]).users.to_set
+  end
+
+  test "finding a room takes the same queries however many direct rooms the account has" do
+    Current.user = users(:david)
+    Rooms::Direct.create_for({}, users: [ users(:david), users(:jz) ])
+    before = queries_during { Rooms::Direct.find_or_create_for(User.where(id: [ users(:david), users(:jz) ])) }
+
+    5.times { |i| Rooms::Direct.create_for({}, users: [ users(:jason), User.create!(name: "Guest #{i}", email_address: "guest#{i}@example.com", password: "secret123456") ]) }
+    Rooms::Direct.create_for({}, users: [ users(:kevin), users(:jz) ])
+
+    assert_equal before, queries_during { Rooms::Direct.find_or_create_for(User.where(id: [ users(:kevin), users(:jz) ])) }
+  end
+
+  private
+    def queries_during(&block)
+      count = 0
+      counter = ->(*, payload) { count += 1 unless payload[:name] == "SCHEMA" || payload[:cached] }
+      ActiveSupport::Notifications.subscribed(counter, "sql.active_record", &block)
+      count
+    end
 end