Honestly only a guess.
I think it matters what Security Hotfixes are installed on the respective machines.
My guess is the slow one is likely the most current in terms of Hotfixes; it is an equally likely proposition it is the least current workstation.
So,
. Make uniform the Security patches and OS Hotfixes for all clients. This may not necessarily mean that your are completely up-to-date. For Security patches listed as "Critical" you must be; for other patches/Hotfixes it becomes a question of what works best in your setting with your applications. Do an inventory of what is installed on your "best" working machine, set it as a standard, and make all other workstations conform in future decisions about applying patches.
You can make the non-compliant workstations "conform" through Control Panel, Add/Remove Programs.
. The second area of importance is the settings under Control Panel, Internet Options.
Look carefully at the settings for the " Local Intranet" Zone under the Security Tab. There may be some Axtive-X and/or scripting controls that differ between workstations. Look too at the "Content" and "Privacy" tabs on your "Best" machine, and having accumulated all the settings make these the standard settings for all others.