Hi,
I migrated from Cubase Pro 9.5 to 10 without major issue. 
I just tested quickly Spectrasonics Omnisphere 2.5 (last update) and the switching between 2 presets is very slow compared to the same use on 9.5. When I click on a patch, I have immediatly the previewed sound but the GUI update of the VST and Cubase are delayed until the complete loading and it take more 5 seconds where on version 9.5 is less 2 seconds (without freezing of the GUI) to switch to another preset. I also tested without my prefs (CTR+ALT+SHIFT) on launch of Cubase and I have the same issue with Omnisphere.
I saw the same issue on the forum in the past on old Cubase versions. Very boring. I come back to Cubase 9.5 for the moment.
For the moment Cubase 10 is not compatible with Omnisphere 2. So not possible for me to work correctly. Hope it will be updated quickly.
No problems here but the latest version of Omnisphere is slower at previewing presets than previous versions - this is also the case in 9.5
In 9.5 loading of a patch is less 2 seconds and very usable. Here in 10, I have more 5 seconds per patch and everything is freez during this time. Totally impossible for my workflow !
It’s nothing like that on my setup. Maybe something is slowing yours down? I’m on a mid 2015 MacBook Pro running High Sierra.
PC I7 4770K + 32 GB of RAM and SSD 512GB on Windows 10 pro last update. No issue on C9.5.
Please verify if your Cubase is not running in compatibility mode.
If it’s the case, untick the option and try once again.
It should be working well now.
I got same issue , after years still can’t solve it
That didn’t help
I recently had the same problem. Omnisphere and Stylus RMX work fine as standalone and in Cubase 11, but run very slowly in Cubase 12. I was able to fix this by turning off some Windows Visual Effects from System Properties - Advanced - Performance - Settings and setting my video card to best performance.