I am not sure where you read that, but only half of it is true. Please point to the documentation and we will fix it.
XDR itself does not run at a lower priority. XDR when used in batch mode makes batch read calls to the local server. There are fewer number of threads which are designated to handle batch read calls. Probably this is what you mean by XDR running at lower priority. However, when you run XDR in single mode, it uses the main transaction threads and you typically have bunch of them (typically 32-64). This mode is especially useful when data is on disk and XDR’s batch mode is proven to be not so useful. Be careful and do homework before switching the defaults.
PS: Amod, if you are using XDR, you must be using the enterprise product. So, its better to get in touch with us directly using the enterprise support channel. We will be able to help you directly.