Thousands of fbx vs uasset super slow

Hi guys, so I have a houdini scene scattering points over a terrain and Importing that into UE4 where the scattered points are bing replaced with trees inside ue4 content browser.

When I try with trees from a ‘forest collection pack’ downloaded from epic marketplace I get great results but I can’t use them in houdini. When I try fbx trees imported into UE4 (and used in houdini) my scene grinds to a halt and I then have to kill the UE4 app.

Does anyone know why my fbx files are so slow when compared to the uasset?

How do I make the fbx files more suitable?