At the risk of appearing to be an apologist, I do think the word 'respond' needs clarification. The CAA have the old ECGs and will have to retrieve them and send them with the new ECG for a cardiologist to read. Consultant cardiologists dont just sit at their desks waiting for pilot's ECGs so the turn around can be some time - our desks are piled high with awaiting reports, but blame HMG's refusal to increase medical school places not the CAA. So the 'response' time is out of the CAA's hands and little to do with them.
The good news is that this is unlikely to be a silent heart attack at your age, although VO2 max has little relevance. Sadly the AME does not normally read the print out so if the little algorithm says fail all they can do is send it off for a real report
I have had several failures myself and if so would recommend you ask the AME:
1 to check the leads are correctly placed - a couple of crossed over leads will result in a real no no
2 the electrodes are not dried out so a proper reading is obtained with no artifact
3 to repeat the ECG - if it then says ok, rip up the rubbish one
Finally always keep your old ECGs - you are entitled to a copy and you just press print twice. If an ECG is sent off and the CAA still play hard ball you can then go see a local cardiologist with no more ado for a second opinion / repeat ECG.