First, TDM 4.3 is already quite an old release and has never been tested on Windows 8 or later. I would recommend upgrading first.
I’ve tried on Win 8.1 x64 VM and the overall behaviour is not that slow, Key or Attribute properties dialogs open within 2 secs at most, but from our experience there are several factors that can deteriorate the performance. Especially real-time antivirus or firewall protection shields can considerably prolong the load times because Windows scripting, used by the dialog forms, is exercised.
There have been several improvements made lately to the forms (the key dialogs were rewritten), so if you want to give it a try, go for the latest Beta release www.toadworld.com/…/default.aspx and see if the performance on your Windows 8.1 machine is better with it.