The GUI >99% of users used to interface with the OS required a GPU to composite the different 2d buffers with fancy effects. IIRC if you knew how do disable as much as possible of it the performance without GPU acceleration was not great, but acceptable. It really sucked when you had an already slow system and the GPU pretended to support the required APIs, but the implementation didn't satisfy the implied performance expectation e.g. pretending to support a feature in as "hardware accelerated", but implementing it mostly on the CPU inside the GPU driver, but even the things the old Intel GPUs really did in hardware were often a lot slower than a "real" GPU of the time. Also CPU and iGPU constantly fought over the often very limited memory bandwidth.