Simple, same problem I had…
If its ETMain then check your using the correct cl_avidemo key and you must make sure that /timescale is set on 1 and not 0. I kept loading a demo, exec a config to reset me demokeys with my own as well as some extra F keys binds yet it would be slow as hell avidemoing regardless of what avidemo I had used. Turns out that config I wrote was /writeconfig while I had reset timescale to 0 somehow. If its ETPro then the same above applies but I would highly reconnmend disbaling b_predefineddemokeys and creating a short config to rebind there F keys.
God its been a while since i’ve posted ere… 