I've seen this discussed before.
I have some pdf's that are not that complex and even after optimizing them they convert extremely slowly with Pdfkit. Attached is two versions of the same document. This single-paged pdf takes approx 40 seconds to convert and it consumes all it can get from my CPU. Just tested setting the resolution to 300 and it resulted in taking several minutes.
Win7 x64 - 8Gb RAM (.Net 4).
Dim myStartTime As DateTime = Now()
Dim myConverter As New Aspose.Pdf.Kit.PdfConverter()
myConverter.StartPage = 1
myConverter.EndPage = 1
Dim myEndTime As DateTime = Now()
Dim myTimespan As TimeSpan = myEndTime.Subtract(myStartTime)