Hello,I use lwcad 5.5 with 3dsmax 2016 and when I add a custom shape in library my viewport becomes very slow and sluggish and unable to work.I' ve noticed that my preset objects are very big 2-7 MB compared to the other presets that comes with the plugin.Why is that?Is there a way to create lighter custom objects ex. windows,doors?Thank you!

It depends on how big object you import into shape library.
Default library objects are optimized, just couple hundreds polygons per object.

If you download some objects from internet, it is usually a big mess.
You should clean it and reduce if possible before you add this to library.

Anyway next LWCAD 2018 will be released soon, and it has a new Window tool, which is specifically optimized for 3dsMax.
New Window is generated only from parameters and it is ultra fast and flexible.

Thanks for your answer.Yes I tried to import windows from my personal library and they are big files.Do you know if I can purchase additional preset shapes?

Which version are you using?
I think we've fixed some problems with large objects in last official release 5.51.

Can you please send me one of your large window object, so I can test with latest build we have?
Just post any support and you'll get reply. Then send me the object by email to that email you get reply.