Really ? I honestly can say I never experienced any.
Guess it depends how you use the phone, what apps you have on there, etc. CM has been a great improvement according to her though, she just had to get used to using the AOSP apps rather than the Samsung ones.
And CM is a massive improvement for older Sammy phones too. The S3 with TouchWiz Jelly Bean is virtually unusable. I stuck CM11 (KitKat) on one for another friend and now it's as fast as my S4 (also running CM of course).
That said, I also know someone with an S5 who has no problems using TouchWiz. But I give it a few updates until it starts bugging out and I'll be asked to stick CM on that too.