Convert the TIFF files to JPEG format. This will reduce the image size, and the amount of data being sent to the plotter.
From my understanding the format won't matter because all compressed image files need to be uncompressed to view/print. If you use JPG the compression is lossy and you'll lose quality once it's uncompressed. TIFF can have different compressions including ones that are lossless.
I could be wrong, though.
Infrastructure Design Suite Premium 2014 Civil 3D 2014 x64 Windows 7 64-bit 6GB RAM