The time increments may not be constant, but the sample rate is more than adequate. Just use both time and value when looking at results. You can't just look at values and assume a constant time increment.

If the total solution wait time is acceptable, then you can keep the 3 mm element. If you want to see results sooner, you can increase this element size.

It is also possible to get an artificially reduced solution wait time by increasing the density of all the materials by a factor of 10 (or 100) which will increase the maximum time step and reduce the solution time, but the results are not the same as when it runs with the correct density. It is useful as a model checkout run.