"Don't want to sound pedantic, but in 1990 I think it was Cat-3, 10BaseT. Cat-4 and Cat-5 were introduced in 1991."
And likewise, it was either Windows 2.0, Windows/286 2.1 or Windows/386 2.0 or a very early release of Windows 3.0, so no built-in networking. As others have mentioned, hubs, not switches were likely the primary cause of the bottleneck and MS had nothing to do with either networking infrastructure or the client network stacks.
Windows for Workgroups didn't appear till about 1992 and IIRC TCP/IP was an optional extra.