Implement The Player Class Through Plug-ins?

I've never actually used Reflection, but I've been warned in the past that using it may cause slow performance. My question is what part(s) of it is/are slow.

I'm thinking about creating a board game and I wanted to implement the player class through plug-ins. That way there could be several different player classes (i.e. human players, and several configurable AI classes). If implemented this way, where should I expect performance hits? Only when initializing the player class? When passing arguments to methods in the player class? In the entirety of execution of methods from the player class? Or is it going to affect the whole application?

The reason I ask is that the AI will be extremely processor intensive. Through testing it seems like it may take around 1-30 seconds to determine a move on my quad core (Core 2 Q6600) processor, depending on the effort spent to optimize the move and testing different branches on separate threads.

My concern is that if I implement the AI via a plug-in, the move determination would slow down considerably to the point where it wasn't playable on lesser hardware.