Try downloading during late-night or early-morning hours (3:00 AM to 6:00 AM) relative to the server’s primary user base (usually US or Central European Time).

Since you have to wait anyway, here is a bit of to pass the time—a look at one of the weirdest "glitches" in nature: The Immortal Jellyfish ( Turritopsis dohrnii )

Fortunately, there are several strategies and workarounds that can help mitigate or resolve the "MountFile all slots for the slow download are in use now please try again later better" error:

Here is a detailed breakdown of why this happens and what you can do about it.