... I am a little bit tempted to move some of the US Large Cap Stocks to International Stocks ...
Yup. Market timing.
15 % of my equities are international. The have under performed terribly. Only recently they are kicking in. ...
This is the way it is supposed to work. Asset allocation, ideally among assets with lower correlations, essentially guarantees that over any short period (years) some classes will be up and some classes will be down. Over maybe a decade, though, the diversification will have reduced portfolio volatility and the portfolio will probably have benefited from each class it turn having its day in the sun. An allocation that is constantly tuned to chase the currently-hot class is essentially a buy-high, sell-low strategy and will almost certainly underperform a portfolio held in patient hands.
International stocks have had a great 6 months. They are up 15%+ while US stocks are only up 9%. ...
Well, you need to be careful here. Internationals looked a little doggy in dollar terms 4Q16 because the dollar was strengthening, not because they were performing poorly. For example, the UK was up 8% but the strengthening of the dollar made it look like zero. I viewed that as money in the bank, since I am long-term bearish in the dollar. YTD17 is a different matter. While internationals have been stronger in local currency, the decline of the dollar has made them look stronger than they really are. That 6% differential is pretty much due to the decline of the dollar. My calculator says the dollar is down against the Euro, for example, by about 8% since the first of the year and down against the GB Pound by 5%. (This assumes that your 15% and 9% numbers are in dollars.)
All that said, there is a Vanguard analysis (
https://www.vanguard.com/pdf/ISGGEB.pdf) that says that 30-40% International is the sweet spot for minimum portfolio volatility. There is also a credible argument, IMO, that since US is about 50% of the world's investable market that, to cover everything equally, a portfolio should be 50% international. Personally, I think we are a little north of 35% right now. I'll be checking again between Xmas and New Year when we take our annual look at the portfolio.
Now back to our regularly scheduled program: To the OP: You may want to reconsider your relatively low 18% allocation not for market timing reasons but rather for portfolio design reasons. If I were you, which I'm not, I would run it up to at least 30%. Same-o for @Blue Collar Guy.
Edit/Currrency reference:
http://www.x-rates.com/graph/?from=USD&to=EUR&amount=1