Both the Express and Premiere only have single band WiFi radios. So they only work on the 2.4 GHz band. While the theoretical speed on that band can approach 300 Mbps, many devices are limited to 100 Mbps or less.
The Roku Stick+ and Ultra have dual band radios, and can utilize the 5 GHz WiFi band. Speeds are usually much higher in that band, partially due to far less interference compared to 2.4 GHz. In the lower band, there's far more competition for the same frequencies.
Dan
Roku Community Streaming Expert
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