I'm seeing the same issue on my 4500SK after applying the new green themed update. And it's obviously not an isolated issue, otherwise there wouldn't be so many of us saying that our boxes are noticeably slower now.
You'd swear that the box is only using 1 CPU core as both input responsiveness and animations are awful now.
I am having the same issues. We have a stick and a box, on both devices it's terrible. The "reaction time" was never great, but now, it's horrendously slow. This update doesn't make the user experience pleasant at all. It's like the software is too big now for the small devices that it's applied to. This really does need sorting out and fast
Why NOW TV had to screw up a perfectly working firmware only they know. The problem is that they are quite happy because this firmware works on Smart TVs, (mine in an 24inch LG), iPhones, iPads and also on Windows PCs. Sadly it does not work on either their set top adapters or a Roku Streaming Stick+. It should be a simple matter to see what is causing the problem by comparing the two versions of the firmware, i.e. the one before 17th March and this latest version. They are making an effort to update the firmware as the latest update is dated just two days ago, V.6.4.3 dated 8 April 2021. However they need a better class of softie!
I can't comment on the performance as all I get is 'SORRY NOW TV is currently unavailable Please try again later.' HA! HA! I have been trying daily since 18 March. I have downloaded and reinstalled the NOW TV app on my Roku stick every day. I cannot do anything about my adapters, Type 4200SK-UK and 4201SK-UK. They just give the same result, blue/green icon, rotating broken circle and then the message. Why, oh Why, did they have to 'update' the firmware. It used work fine.
Are you using any sort of adblocking software on your Router for example Pi-Hole ?
Or you are you using a BT Hub Router, where you need to go into it's Parental file sharing settings to make some changes.
These two above items seem to cause this recent error message.
Type "Sorry NOWTV is unavailable" in the Community search box for various methods how to get rid of this error message .
The firmware was updated to 9.4.3 build 4094-16 earlier this month, but it didn't fix this issue.
Surely they know how to reproduce the issue? It should be obvious since all you need to do is press the home button and then press up or down once. You'll notice that your box won't do anything for 2-3 seconds when switching to or from the home screen. And this used to be almost instant before the green NOW firmware was pushed out to everyone.
Switching between the TV guide, apps and settings seems fine. It's mainly the home screen that causes the box to stall for a few seconds.
The bugs that I've previously reported are all still there too, so I don't know if this will ever get fixed.