It's the difficulty of finding blocks, which makes it harder. Yesterday the 2000 blocks were hit in about 13 hours, because with the lowered difficulty and increased hashrate ~3 blocks were found every minute. Last difficulty change when it was 23k, the average time per block was less than 0.5 blocks every minute, so those 2000 blocks took way longer then the 33 hours. Now the difficulty is 33k, meaning that we miners will spend 3 days finding all 2000 blocks within this difficulty round. Afterwards the diff drops again to sub 13k levels and nicehash will come and f*ck things up again in few hours. This is cancer and as you say the intervals between diff change needs to be adjusted (much shorter intervals), otherwise I am not wasting electricity on Raven anymore and I really like the coin which I am mining since march.