I'm not aware of any good STL to IGES or STEP converters. I'd guess that SolidWorks is converting each triangle (could be thousands) in the STL file to a planar three-sided geometry face and exporting those. As a test, try re-importing that IGES file back into SolidWorks to see if this is in fact the case.

Caedium does not try to immediately mesh imported geometry. I think the slow down you are seeing is because of the large number of individual geometry faces in your file.

STL is a lousy format which we do not support for volume meshing, as I guess you know already - hence the conversion you tried. For more details try the forum post "simulation flow over sailing yacht appendage"

Where did the STL file originate? If it was from a scan then you are pretty much stuck. However, if it came from a CAD package, maybe you can get hold of the original 'good' geometry.

There are packages, such as 3-Matic from Materialize, that can convert scan data to CAD geometry, but they are not cheap.

If you are looking to simulate a fully immersed hydrofoil then you should be good to go with the RANS Flow capabilities. However, we don't yet have a multi-phase (water + air combined) capability.