I hear this a lot, but I really don't get it. The ide is very responsive, when you are interacting with a design, exploring structure or connectivity, static timing analysis, etc.. Perhaps you are referring to synthesising RTL - yes thst is slow, but thats much slower with ASIC tools. Synthesis and PnR is a very cpu intensive task, and is separate from the GUI really. I'd be interested in hearing the specifics of what you found slow.