As I wrote in Plasma NM 0.9.0 release post the main reason for 0.9.0 release was bug #287002 "panel freezes". You problaby have noticed the freezing problem after upgrading to 4.8.0, me too.
The source of the problem is the fact that kded is sometimes frozen by one or more of its modules. Plasma NM < 0.9.0 used to do several synchronous calls to networkmanagement module in kded, specially to get the signal strengh of wifi access points and 3G connections. You can imagine how often wifi access points (all access points in range, even the ones you are not connected to) can trigger signal strengh... signals :-/ Yes, every often.
Any delay in kded causes freezes in Plasma NM's plasmoid and since any plasmoid can also freeze the entire plasma-desktop process the problem is propagated to the entire desktop. I have heard a lot of complaints about kded and Plasma being susceptible to freezes caused by one of their modules (plasmoids in Plasma case), you do not need to repeat them in the comments section of this post. Any Plasma developer is aware of the problem by now, I just do not know if anybody is working on fixing it, so do not ask me.
Several weeks ago I pushed a commit to make Plasma NM use asynchronous API to retrieve data from networkmanagement module. There is also a commit to kde-workspace to do the same for the Solid ModemManager backend, the same change went to QtModemManager as well. This last commit will be in kde-workspace 4.8.1. Those changes seem to easy the problem but, of course, it does not solve it since other plasmoids trying to contact kded will also cause freezes in the desktop.
In bug #287002 it is stated that apper is one of programs that freezes kded. Just as a notice I have never used apper. Until now everybody that has disabled apper's kded module reported that it fixes the freezing problem. You can try it yourself, just go to systemsettings -> Startup and Shutdown -> Service Manager, locate the "apper" service, stop it and uncheck the checkbox in "Use" column for that service. Then click on the "Ok" button.
It is curious that only with 4.8.0 those problems become visible. Plasma NM uses synchronous calls to contact kded since the begining, in 4.4.x time. That is even before I started to use Plasma NM. I guess several other programs do use synchronous DBus API. You can check that by looking for the QDBusReply object declaration and waitForfinished() calls in your source code. If you are using them you should really think about how to avoid them, specially if your program is a plasmoid and/or contact a kded module.