Ah, I didn't think of that. I would say being a full feruchemist would be great for storing attributes, reduced mental speed means you perceive yourself as suffering for less time. unless reduced mental speed reduced the rate at which you stored mental attributes (such as the aforementioned determination) and reduced physical speed reduced the speed at which you stored physical attributes (such as strength). To me, this also brings up the question of whether cadmium and bendalloy in time bubbles change the rate at which you store attributes, because if they do, bendalloy becomes a useful feruchemical tool, and if they don't, cadmium immediately gains another use.
Also, that was a fast quote, it only took me about a minute to perform the edit.