I really really don't get the issue.
Is he io bound? Is there no reasonable way for git hosting services to actually cache git checkouts?
DDoS / DoS should only be used, in my opinion, when the server goes down complelty not if it has a little bit of load and i still haven't seen anything which indicates that this is a real issue?
And i don't want to come across negativly, i just don't get the issue.