So I've noticed this quite some time ago (since 14w11b I believe, although I can't select it in Affected Versions) and I'm wondering if that has something to do with MC-55227. Memory usage cycle is extremely fast in 1.8 compared to 1.7 and I'm attaching two videos for comparison. Also there's noticable drop in FPS, like MC-55227 describes.
MC-55624
Rapid memory cycle in 1.8
Resolved
Won't Fix
6
(Unassigned)
Unconfirmed
Minecraft 14w20a, Minecraft 14w20b, Minecraft 14w21a, Minecraft 14w21b, Minecraft 14w28a, Minecraft 1.8.1
Retrieved
"Works As Intended" / Won't Fix / Invalid
As long as there is no confirmation that this is causing any major issues, this can be resolved. The performance issues are caused by something else entirely.