Well the default is 5 Mbps, and by having your Tablo connected via WiFi you are engaging what I like to call “double WiFi”. That is the stream goes from the Tablo to your router via WiFi, and then goes from your router to the Roku via WiFi. This double trip using WiFi creates a latency which can result in buffering issues.
That is why most of us have our Tablo hard wired to our router. You can hard wire your Tablo to your router using the electrical wiring in your house using Powerline Ethernet adapters, take a read of this thread: