The high RAM usage is due to special caching mechanisms, which make the overall client rendering and loading faster. The client may use up to 1GB of RAM. Also, Java 8 is using more RAM compared to Java 7 (Idk what they've changed, must also be some caching techniques). Please bear in mind that a high RAM usage doesn't mean that something's running slow!!
All in all, I can't do too much to fix this kind of problem for you. I can try removing some caching techniques, but the client's performance will suffer from that. Which would result in a 'low memory' mode... but don't expect that any time soon!