Steemd is demanding on I/O. So, currently you need a fair amount of RAM to keep a full node running flawlessly. I guess the exchanges skimp out nodes for altcoins. Steem has more transactions than Bitcoin and Ethereum combined; and many transactions have walls of text, so naturally it's quite demanding. The developers are working to optimize steemd - it's top priority right now. You can check on GitHub, there's been rapid progress on 0.19.2.