I assume you mean the speed test process and not the simulation process. Either way, if your hard drive and burner have DMA enabled properly, and your hard drive is defragmented, then just ignore the warning, skip the test, and burn your disc.
I just tried the speed test in Nero Express and Burning ROM (they use the same engine) on my computer, and got the same results you did. It complained that it could only write at 24x instead of 48x because the speed of the source data (my hard disc) is too slow. But I assure you that it is not, and that the compilation I tested can be written at 48x with absolutely no problems. I know because I burned this same compilation just a few hours ago.
This would appear to be either a bug in Nero or an excessively over-conservative testing strategy. For what it is worth, Nero took longer to run the speed test alone than it did to read and burn during the actual burning process.