I agree with HD, when ATC get busier, it is far better to speak slower and ensure the relevant info is all absorbed. A rule of thumb is to not pass more than three bits of info in any one transmission, and that should reduce if there is any doubt the pilot would not manage to digest it all. There are many reasons for reducing the amount varying from nationality, quality of the RTF on check in, is "TRAINING" a flight plan remark and emergency traffic. I'd even say "sorry, you were too quick for me there, can you say again" and then the ATC concerned will hopefully deliver it so you hear and understand everything you need.