If your writing curve looks like in these examples:
then it is normal behaviour, the drive slowing down to guarantee low error levels.
The 716 is a first generation 16x drive (actually one of the first dual + and - 16x drives) and as such requires perfect media quality to achieve 16x speed, respectively it uses a very conservative approach when it comes too speed and emphasizes writing quality by slowing down at the slightest whiff of a problem in writing quality. This is nice example how ‘problems’ start to rise towards the outside of the disc, albeit this time without slowing down:
So it comes down a bit to the quality of your media. There are different flavours of Verbatim +R 16x media. Where are yours made? Taiwan or India? Run a FE/TE-Test with Plextools to see how the mechanical qualityies of the discs are. As an example look here how the Focus Error (red line) goes up and the writing speed is reduced accordingly: