Not sure as far as what’s going on with the ext4 setup, but I can tell you from experience - opensource ntfs-3g (as opposed to Tuxera’s proprietary variant) is far more CPU intensive than most filesystem drivers. ext4 should perform significantly better especially if you’re CPU-bottlenecked.

Edit: What protocol are you using? NFSv2 and some NFSv3 configurations do not permit server write caching which utterly kills write performance, especially for smaller writes. Is there a possibility the ext4 partition is mounted with the “sync” option?