FFRend attempts to maintain a constant frame rate, but if the CPU is overloaded, the frame rate will drop, sometimes drastically. CPU overload typically results from a large frame size, a long plugin chain, or especially CPU-intensive plugins. It isn't necessarily a concern when you're recording a finished project. It's more likely to be a problem when the perceived frame rate matters, e.g. during development, or during a live show.

The simplest way to improve performance is by reducing the frame size. You may find it convenient to develop your projects at a low resolution, and then increase the resolution for the final recording, but be aware that some plugins won't behave exactly the same way at different resolutions. If you have a multi-core CPU, you may also be able to improve throughput by load balancing.

The Monitor, Graph, History and Queues views all potentially reduce performance. The monitor quality setting can also affect performance, though only if the monitor bar is visible. Reducing the view frequency may increase performance slightly, at the cost of making the user interface choppier.

If you're creating complex effects at high resolution, it may not be possible to stay in real time, except by using a more powerful computer. FFRend is compute-bound, i.e. its performance is limited primarily by CPU power, rather than graphics power, memory size, or disk speed.