Hi, I have a 6 core. Omnisphere 2,5 spikes on core 1. Any solutions?

There are some patches in Omni that are very CPU heavy. Here are some things you can do to help:
 
1) When using Omni as a multi-timbral synth, add another instance of Omni when the current instance starts to get overloaded. I can usually run between 2 and 6 patches per instance, depending on the patches involved.
 
2) On a really CPU heavy patch, lower the number of simultaneous voices. If the patch uses Arp patterns, try disabling the arpeggiator and see if the patch still works for you. Sometimes, you really don't need as many notes as the arp pattern triggers.
 
3) Disable Omni's FX on patches where you will can add your own later.
 
Regards,
Dan

Make a back of your AUD.INI file.  This is found here (if you've updated to the latest Cakewalk by Bandlab)

%APPDATA%\cakewalk\cakewalk core\

Then go to Cakewalk Preferences | Audio | Configuration file and change the following settings, see it it makes a difference.

MixThreadCount={no of your CPU cores minus 1}
EnableSetThreadIdealProcessor=false

Also, make sure you've disabled core parking. 

In power settings, set CPU min and Max to 100%

Hope that helps.