Potential impact and how to restrict it:
The on-the-fly matrix feature recalculates the actuals matrix in order to ensure the data is up to date before running the Post Timesheets. When we disable it, we need to make sure the Rate Matrix Extraction job with Incremental option is running prior to the Post Timesheets job and also often enough throughout the day i.e. at least every hour if possible. On-the-fly feature is also used for baselining a project or a task, so you want to make sure the rates are always up to date before that, this is why it's important to run the job as often as possible.