Video filters always multiply how hard a game hits your machine but with OpenXcom there should be plenty to spare on any semi-modern computer.
Without knowing more specific information about your computer it's difficult to know why the filters are a problem for you.
Google "how to get a dxdiag file" and attach it. That will tell us pretty much everything we need to know to advise you further. Unless your on a MAC in which case just post your model number and OS version (like OSX Leopard ###). Depending on your hardware and OS I'll have entirely different suggestions and/or follow-up questions.
Otherwise I would want to know what your OS (Windows version) is, your CPU and clockspeed, RAM, GPU, video chipset, current video driver, supported shader model, total VRAM and dedicated VRAM.
Also, unless your are some weird kind of extreme diehard 'X-COM in a Video Filter' fan, I wouldn't recommend dropping Benjamins on a videocard upgrade just for that.