PrimeSeat from IIJ

I’ve managed to get this fantastic (currently free) service from Japan working at DSD resolution (5.6) using a 2016 MacBook Pro and Hugo 2 DAC. However, I only get approx 10 sec bursts of music before it starts buffering (even with a wired (Ethernet) connection). My Plusnet broadband speed is about 30-35 Mbps so should theoretically be sufficient. Anyone have similar experience or know what is wrong? DSD downloads from NativeDSD work fine with Audirvana so presumably the computer is not the bottleneck.