A CPU load >100% in OSX seems to be totally normal on a multicore system.
As each core can have a load between 0 and 100% whereas your OS is simply adding the individual core loads to a grand total.
So for example, a dualcore system can reach up to 200% load, a quadcore up to 400% and a quadcore with hyperthreading support (one virtual core for each physical) up to 800%.
Your macbook is equipped with a hyperthreading enabled quadcore CPU. see: http://ark.intel.com/products/64900/Intel-Core-i7-3615QM-Processor-6M-Cache-up-to-3_30-GHz
Renoise however measures the load differently - more like windows does, where 25% of load on a quadcore means that only one core is at 100% and the others are more or less idling. in this case, OSX task manager would report 100% load and renoise only 25%.
possible causes for your issue:
Check the temperature of your CPU.
The retina macbooks are known to get really hot and even though your book is more or less still brand new, the cooling might be faulty and the CPU therefore overheating.
Halfway modern CPUs (yours) perform thermal throttling as a kind of selfprotection. Throttling will kick in as soon as a certain temperature threshold is exceeded. Throttling would result in a drastic reduction of Vcore and CPU frequency and would therefore logically result in lags and bad performance.
The Tjmax (aka TJUNCTION, aka the temp where your i7 is likely to die) of your CPU is 105°C. If you’re remotely near that temperature, throttling is very likely to kick in. i’m not sure when exactly it’ll start to throttle unfortunately, but i guess anything above 80°C would get things rolling…
I’ve read about similar cases (lags, bad performance) where the cause was a faulty battery. performance came back as soon as the battery was removed or replaced by a new one.
i never had a mac nor will i ever have one, so it’s just blind guessing