> Internet Explorer had a number of iffy behaviours and lagged behind other browsers for many visual effects, even regarding basic properties like opacity. Thus, Microsoft brought in bespoke filter functionality.

That's not accurate at all. At the time IE4 launched, it was ahead of the competition. It also had quite a lot of non-standard extras as part of Microsofts strategy to embrace, extend and extinguish. IE4 was extend. In other cases the strategy worked, but not for the web.

The DirectX filters weren't used pretty much anywhere until it turned out it could do things that were specced and implemented by others years later.