I don't know the answer but, in my day the Clearance Delivery Officers at Shanwick were ATCOS &, if you were requesting clearances within VHF range of Prestwick you were talking directly to them. They had the necessary info. at their fingertips & were able to issue clearances pretty much instantaneously. On HF you are (& were) talking to Radio Operators in Shannon. They only relayed messages by telex to & from the ATCOS at Prestwick. So that accounts for delays to issuance of clearances on HF. I don't know what the mechanics are, specifically, nowadays; but I understand that you can still request & receive clearances by VHF & HF. I don't know if you are talking directly to ATC anymore. I also know that many a/c use CPDLC. I would have thought that, in both cases your requests would be dealt with by the appropriate ATCO & not suffer any delays such as are likely on HF.
I don't know what Gander does, but I would have thought that it was much the same. So, the equipment & procedures used for the request may have changed since my day & there is certainly a lot more traffic now. But, I don't know why there should be this discrepancy between Gander & Shanwick.