On the NFC Shield V2.1, PN532 pin 37 SVDD is connected to the 3.3V power supply (U4 regulator output). I believe this is wrong because SVDD is an output from the PN532 chip and is intended to supply a secure core if one is used.
I cut the track connected to pin 37 (where it connects to C2) and made some current consumption measurements of my system before and after the modification.
(My system is an Arduino Uno R3 + Seedstudio NFC Shield V2.1 + bespoke shield with RTC module, buzzer and miscellaneous logic).
Current consumption before mod: idle 310mA, during NFC tag read 270 to 290mA
Current consumption after mod: idle 250mA, during NFC tag read 190 to 210mA
Please note cutting the track might not be the best course of action. I made the same mod to 4 more shields that I had bought from Seedstudio and one of them stopped working after I cut the track. I suspect the PN532 internal regulator had gone faulty and therefore was relying on the external 3.3V being fed back into the output pin to power the chip. Re-soldering the track was very tricky.