https://github.com/cpu-pool/cpuminer-opt-cpupower/releases
cpu affinity = 0x55 = 0 2 4 6
ok, i have an i5-2500k , it has same instruction set as your cpu, but 4 threads.
With all 4 threads i get ~430hs, sse2 cause no sha available. No affinity set, just --cpu-threads 4 .
I tried the miner from your link , 4 threads give approx the same result.
I will compare these two on a few more machines, but i think there won't be such a drastical difference as in your case.
How does your cmd line for SRB looks like ? Are you using the same affinity settings as with that other miner?