Problems importing files into Freedom Studio project

I am attempting to use a Freedom Studio (under windows) / Arty A7 board combination to perform some performance profiling on an existing software package and am encountering a couple of problems. I have taken the basic sifive-welcome demo project and am trying to add my own files to this.

When I import files (using the file -> import -> File System), they appear to be being brought into my project, but they are not being included in the build. I have temporarily worked around this problem by adding my files to the Makefile but this is not really satisfactory.

Also, the resulting build is very slow (10 minutes for 130 source files).

Not sure what is the problem. I create dummy 150 source files, each has 20 functions. Import take about 10s and build take about 16 seconds.