Question: "If you change the enchantment glint on rp to a plain colour, or just nothing, does that reduce lag?"

Answer: "From what I know, you have to remove it from rendering. Changing the texture wont do anything, it still technically renders even though you can visually not see it."
-Brottweiler (Overcast Network Moderator)

I have seen several videos advocating the use of transparent glint.png textures to 'fix' the lag on enchanted bows while drawing them back. This does not work because the game still has to render an invisible image. Instead, you must use a mod that stops the game from even rendering the glint on your held item.

Here's the oc.tc forum thread: oc.tc/forums/topics/51dc4a56ba6087ea1100252b
and download of Guru_Fraser's mod for 1.6.4: www.mediafire.com/download/2rcr2oxkk97htkn/bfj.class

Another similar mod with additional features you may or may not want:
www.minecraftforum.net/topic/1904538-enchanted-item-lag-fix-more-fpsplus-updated-as-of-11-27-13/

Make sure you download the FPSPlus+ version, as this one removes the enchantment glint entirely.