Flow simulation takes a long time

look at the task manager and how much memory is being used, compare against your installed ram

When you load the results there will be a topic underneath (cut plots, surface plots, etc.) that is Goals Plot.  If you select this it will make a plot of your goals vs (iterations or travels or time).  You should also be able to RMB on the Load Results and select summary, I believe this tells you.  Or when you load a set of results the right side of the pane will show the iterations.

I have a large heat transfer model that has certain areas at room temperature and others at 1000C+.  If you set a single fluid temperature to start it has to iterate long enough to raise or lower the local temperature.  But if you make a volume that represents a certain region you can apply an Initial Condition for the gas in that area.

Example:

I have a 10ft long tube that has 0C gas entering one end.

Heat is applied to the outside of the tube.

The gas exits around 1000C (based on a previous long simulation solve)

If I break this up into 1ft sections where each one is approximately 100C hotter than the last (or whatever the previous results indicate).

Make a cylinder the ID of the tube and put it inside as a solid plug.  Repeat this along the length (likely with a linear pattern in this case).

Choose the first plug and set an Initial Condition.  Click the box to "disable solid body".  Flow will now only use this body to define the conditions of any point inside of it, but it will be hollow (fluid).  Set this initial condition to 100C.

Repeat this step for each volume but setting the temperatures of each one hotter than the last.

Now when you run Flow will start with a temperature progression instead of assuming a single uniform temperature.  The more complex your system the more this helps.  Especially if you have forced flow in areas resulting in rapid heat transfer but natural convection in others which can take forever to converge.

how fast do you want your simulations to be exactly?