The issue is that the value isn’t called, “LinkSpeed” the value is called, "Link Speed " with a space after speed.
You should either fix whatever sets this registry key or just use:
Q: (value "LinkSpeed " of key "HKEY_LOCAL_MACHINE\Software\BigFix\LinkSpeed" of registry) = "555k"
If you do what you just previously mentioned (just value of key) it will throw an error if you ever put another value there and it will catch a value with any name not just linkspeed.