None of the current gpu mineable algos are cpu intensive, there's absolutely zero difference between i5 and the cheapest celeron for Ethereum mining. Sometimes (very rarely) a new coin shows up with a completely new algo, and the very first miners for that coin might not yet be optimized so people with higher-end CPUs might have an advantage for a very short period of time (before a properly written miner shows up). I think the last time it happened it was sibcoin (not 100% sure). Then again, you'd probably want more than 4 threads for those poorly written miners (8+). These situations are so rare and you're most likely not even gonna hear about those coins well until they've matured and there are no high cpu load issues with the miners anymore, so it's just not worth it to bother with any of this stuff. Most people use low-end cpus in their rigs and do just fine. I've got both two-thread cpus (celeron g3900/3930), 2core/4thread cpus (g4560) and even 4core/8thread i7-6700 in one of the rigs — no noticeable difference between any of these.