Most people I talk to notice that Windows 8 and 8.1 are actually faster than Windows 7. Having never extensively used Windows 7 myself, I can't say. If you were using a 64-bit version beforehand and are somehow using a 32-bit version of Windows 8.1, that would explain the difference.
Also, if you happened to have used an older version of Dolphin (3.0-xxx or 3.5-xxx) and are now using a newer version (4.0-xxx) that could also explain the speed difference, although Dolphin should be pretty fast in the latest development revisions. In your case, however, it might not be fast enough to make up for the "slowdowns" in recent changes however.
Anyway, your system's bottleneck is the CPU. Your CPU, though clocked fairly high, is outdated by several generations of Intel hardware. Sandy Bridge (one generation after Westmere, where you i5-655K falls in) really packed a noticeable punch when it came to Dolphin, and Haswell now does the same (Haswell is usually 20~30% faster than Sandy or Ivy Bridge when it comes to Dolphin. We've benchmarked this several times). In short, your CPU is showing its age.
Also, if you happened to have used an older version of Dolphin (3.0-xxx or 3.5-xxx) and are now using a newer version (4.0-xxx) that could also explain the speed difference, although Dolphin should be pretty fast in the latest development revisions. In your case, however, it might not be fast enough to make up for the "slowdowns" in recent changes however.
Anyway, your system's bottleneck is the CPU. Your CPU, though clocked fairly high, is outdated by several generations of Intel hardware. Sandy Bridge (one generation after Westmere, where you i5-655K falls in) really packed a noticeable punch when it came to Dolphin, and Haswell now does the same (Haswell is usually 20~30% faster than Sandy or Ivy Bridge when it comes to Dolphin. We've benchmarked this several times). In short, your CPU is showing its age.