If you start your task with 'thin' disk, you'll not be stopped to start conversion - wait to see the transfer rate for few percent and then cancel the task.
You are correct about Converter view of disk resize, but if you optimize your disk, then you can decrease partition type from OS (use DiskManager to shrink the volume size). Then you should use some of vmware-toolsXX to shrink the size of VMDK. With such big disk I'll say that transfer speed is lowered because files are read one-by-one and written one-by-one, that's because there is a warning about this type of transfer.