I don't remember installing this DDraw fix, maybe that is the problem unless that is part of one of the other mods.
ddrawfix is now installed by default in all GOG's IE games & it may well be the problem.
I haven't used BGT since GOG started using this, but with TuTu I uninstall it before instaling the fixpack or TuTu.
I've found ddrawfix actually stops a slow down I used to get when fighting dragons, so I reinstall it just before the widescreen mod.
If I were you I would uninstall all your mods except ddrawfix & start a standard bg2 game & try some spellcasting.
If the problem is still there, uninstall ddrawfix & get the problem sorted in the standard game before modding it.
If the standard game runs fine with ddrawfix installed, I would uninstall it, install the fixpack & BGT & then reinstall it.