The editor is busy compiling textures - this is not Mac specific. Compressing textures using DXT is processor intensive & newer PCs will have significantly faster CPUs than your , ergo they will take less time to compress textures. On OS X the compression process is also fighting for CPU time with our shader compiler that translates HLSL to GLSL as required on OS X. Windows doesn’t have that extra overhead when using D3D as the HLSL can be used directly so there’s more processor time available for texture compression.