The problem is not with dvd2oneX. It is with MacOS X and it's disk caching algorithm.
When you access large disk sections as you do when writing a 4GB DVD most operating systems use unused memory to cache that disk section in memory to increase performance.
Unfortunately Mac OS X seems to have an algorithm for this that is too aggressive against running applications. That means that it will page out to disk your running, but not active, applications to gather more free memory. This means that when you try to do something with these applications that they need to be paged back in from disk. This paging is extremely inefficient and cause horrible lagging and the performance impact you have seen.
Unfortunately I have not found a solution to this problem.
There are a couple of work around to this. One is logging out and in forces all your applications to be terminated and the new applications started won't be paged out.
Not a lot of help I know but hopefully you now understand what id happening.