TFTP is crazy slow, even with RFC 7740 (buffering), but the payloads are usually small so few people care.

Thankfully modern BIOSes tend to implement HTTP boot option, where you can point to any HTTP or HTTPS URL (as long as the URL ends with ".efi", which is a pretty dumb limitation if you ask me).

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.

They let you boot off HTTPS? That explains why corp IT pushed out a Dell BIOS vulnerability update today relating to OpenSSL in my BIOS.