When compared to the rest of the iMac lineup, the new iMac has reasonable single-core performance — it's almost identifcal to the entry-level quad-core iMac. Multi-core performance is significantly lower due to the lower number of cores (2 cores vs 4 cores).
One interesting thing about the new iMacs is that they use a low-voltage i5-4260U "Haswell" processor (the same processor is used in the MacBook Air). Why would Apple use a low-voltage dual-core processor in a desktop machine? The answer might be graphics:
I'd say Apple used an Ultrabook CPU in the new iMac to avoid shipping Intel's HD 4600 GPU in anything. No desktop CPUs include HD 5000.— Andrew Cunningham (@AndrewWrites) June 18, 2014
According to Intel, the HD 5000 is twice as fast as the HD 4600. Apple may have sacrificed multi-core performance for GPU performance. Given the increasing importance modern user interfaces place on GPU performance, this may turn out to be a smart decision that extends the useful lifespan of the new iMac.