Cakewalk and Sonar Platinum have a problem for scrolling speed.
If a MIDI track had a lot of control data, the speed of scrolling & shortcut keys & drawing & whole operatings become slower.
I test it many times in Cakewalk Bandilab and several Sonar versions, with Intel 8700K GPU and NVIDIA GTX1060 VGA.
I attach its video.
 
     http://www.midist.pe.kr/temp/scroll.avi
 
PS. There is no this problem in Sonar X3.
 
 
 

Yes! Confirming the exactly same issue in Platinum Win7 32-bit.
 
Even if single MIDI track with loads of just one continues controller data and scrolling, editing, even making just one small single MIDI event edit takes ages! I reported it back some time ago, had a response about 6 months later that they improved the whole MIDI engine in Sonar (made the issue behave just slightly better, not much really). Couple weeks later Gibson announced close-down.

I just found that the whole operatings become slwoer upto not work anymore. It's very serious...

midist
...the whole operatings become slwoer upto not work anymore. It's very serious

 
Agreed! I have to wait something like 20 seconds for a single MIDI event edit to take place. Very annoying.
 
If this issue is still present in the new BandLab version, maybe it's worth reporting to their new Cakewalk support.