Window mode will ALWAYS be slower than full-screen mode even if it's windowed full-screen. You can lose upwards to 30fps on some games just for being in window mode.

In BDO there's a setting that may not be in the same submenu that forces FPS lock. However, if that's not the case and if you're using nvidia you can open the nvidia control panel and confirm your settings there. Make sure that your native resolution and Hz is set correctly and then when it is, you can go to GeForce Experience and configure BDO from there and override BDO settings. They also have their own injector for some games, I don't know if it has one for BDO. Not like BDO needs one but if it applies one on top of what is already there, maybe it could affect performance? AMD has its own thing and I don't know what it uses but it's probably a similar process.

And just to be sure, you're not playing on the best graphical setting right? So in every mode it has that FPS lock?