[popcornmix]

However, I'm still wondering if it's normal to see [Buffering...] for a few seconds when seeking around very large files. I'm using wired gigabit Ethernet and NFS. My over HTPC seeks around pretty much instantly.

EDIT: Also, I am using a 64 GB Samsung microSD card because I didn't have anything else available. I've ordered the NOOBS card directly from RPi foundation because I heard that's a very good card, but could my existing card be causing some of this buffering?

mkv or iso? mkv files are a bit quicker to seek.
I wouldn't expect the sdcard to affect buffering time (unless using buffermode=1 and cachemembuffersize=0 which would use the sdcard).
The network speed will affect it. May be worth checking with iperf from server to Pi. Should get around 90Mbit/s if all is good.