I'm not sure what kind of juice is getting put in there, my shaders now execute much better on IE11 than Chrome and Firefox.
IE probably uses DX11. Afaik Angle (currently) still uses DX9. The DX9 HLSL compile process contains the Microsoft D3D compiler chain, which is very slow. It's fair to assume the DX11 compile process has been optimized. I remember Microsoft now also favors delivery of HLSL rather than intermediary bytecode. It's for this reason that games in the past often built their own compiler to intermediary bytecode, but starting with DX11 Microsoft requires that any intermediary bytecode is cryptographically signed by the D3D compiler, and so it's probably more important that it performs well in DX11.