IIRC, the boot.wim is downloaded via TFTP and the performance can be affected by the TFTP Block Size. There are two places to configure the TFTP Block Size. I believe the one exposed in the OSD Web Interface affects the transfers up to starting the Windows Boot Manager (so, basically, just getting as far as the rembo menu on the client, I think.) Then there's a second TFTP Block Size option used by the Windows Boot Manager to download the boot.wim file; I believe this one can be configured by a task in the OS Deployment and Bare Metal Imaging site, look for fixlets/tasks with TFTP in the name.
The actual value that's optimum depends on your specific network, how many hops to the client, MTU sizes, and expectation for dropped packets. I'll try to lookup my value next week, but I think that at least 1434 bytes is good for matching Ethernet MTU size and you should go at that size or larger (and in multiples of 1434 assuming Ethernet with MTU 1500 bytes). I'll correct this post next week if that's the wrong value.