Did you wire this machine to the network? If you had it using WiFi and your Steam Link on WiFi, that's pushing the overall WiFi speed down.
Get all wired for a test.
I have a very old CPU Intel Core Duo E8500 @ 3.16 GHz and ASUS P5Q Pro motherboard. Streaming on steam link to my bedroom tv is very slow with slow encoding always displayed when 'Display Performance Metrics' is set. I am playing strategy games like XCOM so my desktop computer as such satisfies all my needs when I play directly on it. I ran a performance log on my desktop while playing on steam link my cpu is at 100% usage while streaming and I am only getting 10 fps even at 720p resolution. I was wondering if overclocking my CPU to 4GHz will help. If not should I just go for a quad core processor 'Q9700 extreme edition' or 'Q9650' will it give me at least 30 fps or is my only option to upgrade to a brand new CPU and motherboard?