Hi, I had a similar problem after updating to ios 8 on my iphone 4s. It seems like with ios 8's new feature of being able to reply to notifications "in-line" (like with message notifications) the ios 8 notification handling framework has become more complex and a bit of a burden for the earlier A5 processor.
I fixed the problem on my phone with the following procedure:
1. Launch settings and scroll all the way down to the bottom where the installed apps are listed.
2. Tap on an app to enter settings for that app then tap "Notifications".
3. Turn off the "Allow Notifications" switch.
4. Exit back out to the app list and do the above 3 steps for each app.
After doing this my iPhone 4s returned back to being as fast and responsive as it was before the ios 8 upgrade. I did not have to mess with transparency or animations or anything else.
Hope this helps!
Rob