Slowdowns are never intended, if it's "constantly slowed down" it was because the routine took too long on some model and you went for half framerate instead.
The usual source for intermittent slowdowns is the coder coding for a more powerful model and hasn't added code to manage framerates.
This sometimes happened even for base models, if they were made more powerful by adding true fastmem (faster than chipmem) and never thinking of that they might end up in slowmem (as fast as chipmem).
Sometimes slowdowns are due to system interrupts still running. If it gets worse if you wiggle your mouse, that's the source for it.