Remember, input lag is present with VSync on, as documented and described in detail on the bug tracker. If you want to have a somewhat positive game experience in an action game in Nestopia, I recommend disabling VSync until the bug is fixed.
I am going to respectfully disagree with your above statements. I am not denying what you, the individual who posted the "bug" or anyone else who plays Nestopia experiences. Nonetheless, it is my experience that there is no noticable input lag. I play with VSync on and the Super Mario Bros. series, Castlevania series, Rygar, Spelunker, Rush 'n Attack, Elevator Action, Gyruss, and other titles all play without issue including no input lag.
If you have a specific game in mind when referencing "action game(s)" in Nestopia which I haven't already mentioned, I would be happy to try it and see if I notice any difference/lag in input response. I also compared with Nintendulator and found the response time and reaction the same.
Again, this is *my* experience using Nestopia. Sorry to read some users are having issues, but I would recommend for each user to try it for themselves. My action gaming in Nestopia is nothing but positive.