HQxx filters are extremely CPU tasking, and even the most high-end computer might have trouble running them. On the flipside, considerably lower-end machines can run even HQ4X without even losing a single frame. I remember MaxSt once telling me on IRC once that the fluidity of the filter depends heavily on the bandwidth and the bus between the CPU and the GPU (or something of that sort anyway, I don't remember since it's been a while). Personally, I've never got any HQxx filter to run smoothly on my machine, but at the same time I know people who get constant framerates with HQ3X with a Geforce 2/3 equivalent card. Do note that my observations are based on ZSNES, not MAME but the filter is the same. Yes, HQxx filters are the best if you want a sharp picture, the like of [super] 2XSaI and Kreed's Supre Eagle are a lot faster, but they're additive blur filters.