In assessing the playability of a diatonic button accordion, I consider the accordion's "quickness" to be one of the most important factors.
Years ago, I believed that the speed of the keyboard's action was the key element that determined the instrument's quickness. Some buttons feel hard to press down, but have a quick return; others can be pressed effortlessly, but seem slow to return the pallets to the action board. It seemed to me that a well-fettled accordion would achieve a balance in spring strength that would render the box "quick."
Now I believe that the keyboard's action is important, but not the most crucial contributor to the impression of a box's quickness.
Reed response is critical. An accordion with reeds that respond instantly to small amounts of airflow will render an accordion fast enough to play, say, a French Canadian reel, regardless of how sluggish the keyboard action feels.
The quality of the reeds plays a huge role in reed response, but even lower quality reeds can be adjusted by a skilled accordion tech to achieve a very high level of performance. I met French Canadian accordion builder Marcel Messervier in 2002, and he told me that he spends a lot of time adjusting the set of each reed in order to achieve the "nervous" (his word!) response that Quebecois players demand. Depending on the players preferences, builder Sylvain Vezina (Accordeons Melodie) will choose longer scale (quicker but quieter) or shorter scale (slower but louder) reeds.