You can also do things like boot with PXE (Legacy or UEFI PXE boot) to get a small image like iPXE, and then have iPXE do the http boot part. This means that you have an extra shim but you can pull larger images than TFTP is any good for.

TFTP is also UDP and I don't think it is pipelined, so it's all req->ack->req->ack, so any additional latency hits it hard too.