[Sholander]

The picture (video) lags somewhat behind the sound, to the extent that it's unwatchable.

RPI is not powerful you can limit the resolution to 720P in expert settings    

I've limited it, but the video stutters quite badly. Worse than it did before I last upgraded the add-on I would say. 

Don't know how it is on RPi, but on Amlogic boxes NF addon works perfect with previous Widevine version. On Amlogic boxes current libwidevinecdm.so is v. 4.10.1440.18 with which streams stutter a lot. Replacing libwidevinecdm.so with previous v.4.10.1303.0 solves the problem and NF addon works as before.
If you have a Kodi backup previous to widevine change, then extract the libwidevine.so from it and replace the present one to try if it also fixes the problem on RPi.