YM (and ES) are VERY THIN at that time of morning. In the case of YM, a single-digit number of contracts can and does move the market.
That said, I agree with Mandlebrot... look to Europe. I prefer the FESX; same increment as YM (1 tick = 1 pt), but each tick is 10EUR, or approximately $15US after conversion. Daily volume is roughly the same as ES. In other words, highly liquid. Between 12 and 1pm EST Europe winds down and trading starts to thin. Another nicety is many brokerages provide reduced intraday margin, which kicks in between 8 and 9am EST; depends on the broker however.
One more nicety... AT WORST round trip all-in is comparable to ES. Frankly, IMO, if you pay that much you are paying too much!
A few questions to be asked of YOUR broker... how is currency conversion handled? Are there associated currency conversion fees? Is value credited/debited in real time? Is the value official or interim? When is an official conversion price determined?
Good trading