1.5 seconds should be much longer. The new "feature" is clearly there to make people accidentally spend diamonds because there's no need for it. You could already click the + button and buy KP with diamonds that went directly into the AW.
This is consistent with the KP bar interface. Inno deliberately broke this consistency and made the interface less logical and thus harder to understand. Less comfortable even if they claim the opposite. I can only guess the motivation behind the change, but it has to be very important to Inno.
The 1.5 seconds delay is probably all we can expect