Go into the BIOS and look at the disc caching options. HD write cache (can't remember the exact name) needs to be enabled, which it isn't by default. You're seeing the hard drive buffers filling up, and ESXi being a server level VM manager will wait until it receives the all clear that everything has been written before sending more data. Enabling the cache will make this *much* quicker. Only downside is that if there's a power cut when writing, you'll lose data. I found this issue myself, bought new network card and the like and then stumbled upon the BIOS setting and it flew afterwards!