I've read reports, advising to hard-set the switch port that your Spark connects to, to 100 mbit full duplex (instead of to Gigabit or to Auto sensing; the Spark's ethernet port is 100 mbit, since the bitrate of NDI|HX is around 10 mbit/sec).
In my case (Netgear GS728TP managed switch) the reverse turns out to be true:
hard-setting the port to 100 mbit full duplex gives terrible latency and dropped frames;
setting it to Auto solved that problem. Even though in both cases, the switch reports the same "physical status", i.e. 100 mbit full duplex.