You have to be careful using the same mechanics for a slow spell (a game effect) and a bog (a terrain effect).

If you have any pathfinding, you will have to iterate over terrain’s move cost without considering the slow spell.

Plus, I’d rather have the game content (the slow spell, the bog terrain, the heavy armor, etc.) separate from the timing / ticks / etc. You want to be able to tweak content and add content easily without breaking the core game.

Just my 2 cents.