Tl;dr You are creating a new Texture2D instance (1x1 pixel) called dummyTexture on each call to the DrawMiniMap method. This is a bad idea, as textures need to be sent to the GPU every time (and then disposed, and collected). Add a field named dummyTexture and fully initialize it inside LoadContent.
But please read the rest of the post to see how to identify these issues.
Your Update method is being called at 60fps, but this doesn't have to be true for the Draw method too. Since Game.IsFixedTimeStep is set to true by default, as soon as Monogame sees that Draw is taking longer than expected to finish, your Draw method will skip frames.
Due to the way your game logic is written right now, this is the only way for Monogame to ensure that all updates are done in fixed time steps.
To get better insight into what's happening, first create an additional set of fps variables for Draw method fps, and render both values on the screen. You will see that Update is drawn at 60 fps, but Draw falls below that.
protected override void Draw(GameTime gameTime)
{
// similar to what you have inside `Update`
drawFpsTimer += gameTime.ElapsedGameTime;
drawFpsCount++;
if (drawFpsTimer >= TimeSpan.FromSeconds(1))
{ drawFpsTimer = TimeSpan.FromSeconds(0); drawFps = drawFpsCount; drawFpsCount = 0; }
...
With this in place it's easy to pinpoint the problematic method inside Draw, and get your fps back to 60.
Also, by setting IsFixedTimeStep to false you can get both Update and Draw to be called in succession, but your Update frame rate will drop too - meaning you should change all your update logic to use gameTime.ElapsedGameTime instead of presuming fixed steps:
protected override void Initialize()
{
// use variable time step
this.IsFixedTimeStep = false;
this.TargetElapsedTime = TimeSpan.FromSeconds(1 / 60.0);
this.graphics.SynchronizeWithVerticalRetrace = true;
...
I prefer variable time step in all my projects, especially when you find out that XNA sometimes takes up 100% CPU time on one core when the default value of Game.IsFixedTimeStep is used, but get ready for some refactoring then.
As for why the frames are being skipped, it seems it's the DrawMiniMap method that's problematic. Commenting this out gives 60 fps update + 60 fps draw rate on my machine.
Upon further inspection, it turns out you are creating a new Texture2D instance (1x1 pixel) called dummyTexture on each call to this method. This is a bad idea. Add a field named dummyTexture and fully initialize it inside LoadContent. You could also slightly speed up this method by making tileColour a member of the Tile class to avoid a bunch of if tests on each iteration, but there are probably many optimizations like this and are perhaps not needed unless you are deploying to mobile devices.
