4x -> 6x -> 8x change is fine. It is supposed to do so.
I remember 8x(in fact 4x->6x->8x) burning takes a little bit less than 10 min.
If your burning really takes more than 10 min, check the burning speed information
if there’s anything weird happening.
It should start at very low speed, like 0.5x, and then in a few second you have to observe 4x, then it goes to 6x (at the transition you may see something like 3x or 5x, but don’t worry, it should disappear in a few second), then 8x transition happens. At the last stage, there’s an additional step of leadout(or whatever the right term) that takes about 10 sec. Then the burning is over. So, unless you see some weird behavior (for example burning speed drops to 4x permanently at the end of a disc) apart from this, everything is fine.