Feature: Framerate display window (#6822)

Frame rate and various game loop/graphics timing measurements and graphs. Accessible via the Help menu, and can print some stats in the console via the fps command.

README.md

@@ -14,6 +14,7 @@
     - 4.4) [Files in tar (archives)](#44-files-in-tar-archives)
 - 5.0) [OpenTTD features](#50-openttd-features)
     - 5.1) [Logging of potentially dangerous actions](#51-logging-of-potentially-dangerous-actions)
+    - 5.2) [Frame rate and performance metrics](#52-frame-rate-and-performance-metrics)
 - 6.0) [Configuration file](#60-configuration-file)
 - 7.0) [Compiling](#70-compiling)
     - 7.1) [Required/optional libraries](#71-requiredoptional-libraries)
@@ -433,6 +434,70 @@ No personal information is stored.
 You can show the game log by typing 'gamelog' in the console or by running
 OpenTTD in debug mode.
 
+### 5.2) Frame rate and performance metrics
+
+The Help menu in-game has a function to open the Frame rate window. This
+window shows various real-time performance statistics, measuring what parts
+of the game require the most processing power currently.
+
+A summary of the statistics can also be retrieved from the console with the
+`fps` command. This is especially useful on dedicated servers, where the
+administrator might want to determine what's limiting performance in a slow
+game.
+
+The frame rate is given as two figures, the simulation rate and the graphics
+frame rate. Usually these are identical, as the screen is rendered exactly
+once per simulated tick, but in the future there might be support for graphics
+and simulation running at different rates. When the game is paused, the
+simulation rate drops to zero.
+
+In addition to the simulation rate, a game speed factor is also calculated.
+This is based on the target simulation speed, which is 30 milliseconds per
+game tick. At that speed, the expected frame rate is 33.33 frames/second, and
+the game speed factor is how close to that target the actual rate is. When
+the game is in fast forward mode, the game speed factor shows how much
+speed up is achieved.
+
+The lower part of the window shows timing statistics for individual parts of
+the game. The times shown are short-term and long-term averages of how long
+it takes to process one tick of game time, all figures are in milliseconds.
+
+Clicking a line in the lower part of the window opens a graph window, giving
+detailed readings on each tick simulated by the game.
+
+The following is an explanation of the different statistics:
+
+- *Game loop* - Total processing time used per simulated "tick" in the game.
+  This includes all pathfinding, world updates, and economy handling.
+- *Cargo handling* - Time spent loading/unloading cargo at stations, and
+  industries and towns sending/retrieving cargo from stations.
+- *Train ticks*, *Road vehicle ticks*, *Ship ticks*, *Aircraft ticks* -
+  Time spent on pathfinding and other processing for each player vehicle type.
+- *World ticks* - Time spent on other world/landscape processing. This
+  includes towns growing, building animations, updates of farmland and trees,
+  and station rating updates.
+- *Link graph delay* - Time overruns of the cargo distribution link graph
+  update thread. Usually the link graph is updated in a background thread,
+  but these updates need to synchronise with the main game loop occasionally,
+  if the time spent on link graph updates is longer than the time taken to
+  otherwise simulate the game while it was updating, these delays are counted
+  in this figure.
+- *Graphics rendering* - Total time spent rendering all graphics, including
+  both GUI and world viewports. This typically spikes when panning the view
+  around, and when more things are happening on screen at once.
+- *World viewport rendering* - Isolated time spent rendering just world
+  viewports. If this figure is significantly lower than the total graphics
+  rendering time, most time is spent rendering GUI than rendering world.
+- *Video output* - Speed of copying the rendered graphics to the display
+  adapter. Usually this should be very fast (in the range of 0-3 ms), large
+  values for this can indicate a graphics driver problem.
+- *Sound mixing* - Speed of mixing active audio samples together. Usually
+  this should be very fast (in the range of 0-3 ms), if it is slow, consider
+  switching to the NoSound set.
+
+If the frame rate window is shaded, the title bar will instead show just the
+current simulation rate and the game speed factor.
+
 ## 6.0) Configuration file
 
 The configuration file for OpenTTD (openttd.cfg) is in a simple Windows-like