I'm not sure when you tried. The current state-of-the-art emulation for melee is much more responsive than console melee; on a high-refresh-rate gaming monitor, you have 2 whole frames of extra input lag to play around with. For local play you can just set the appropriate buffer to match console melee; if you're playing online, those frames are used to send inputs to your opponent(s), making online possible without any extra delay.
Mmmm, if they're trying to play on a Mac, Slippi is currently a bit rougher than the other platforms. A common misconception is that Slippi = Dolphin; in reality, Slippi is (currently) a fork of an older Dolphin and doesn't have the features for macOS that modern Dolphin has (e.g, a Metal/MoltenVK graphics backend). OpenGL on macOS is a nightmare - and indeed, if you opened mainline Dolphin right now and tried to play it using OpenGL, it will tell you in bright yellow letters that it's going to be bad. :)
I backported an earlier version of the Metal/MoltenVK backend to Slippi which should hopefully be in the next release, whenever they get around to it. This provides decent performance on Intel Macs and ARM Macs under Rosetta 2, and ideally tides the (mac) community over until Slippi finds parity with mainline Dolphin.
