TCP slow start is not an issue for load balancers, as operatings system cache the congestion window (cwnd) on a per-host basis, even after termination of all connections to that host. That is, next time a connection to the same backend host is created, the OS uses a higher initial congestion window (initcwnd) during slow start based on the previous cache value. It does not matter if the target backend host is in the same datacenter or not.