We did this in January and yes traffic can be bad, but it is all freeway driving and can expect to be about 1.5 to 2 hours to IAH. we did
DCL transfers and we were at the airport before 11 (off the ship at 9). But given you have a 5pm international flight you probably do not need to be there until 2pm and depending on the carrier, may not be able to check your bags prior to 2pm. There isn't much to do at the IAH airport outside of security, unless you get a day room at the marriot (expect about $100 for a room). So you probably would be safe to leave Glaveston at 11 or noon to get to IAH.
There are lots of shuttles services between Glaveston and the airports... But some of them make multiple stops so besure to check on that if you end up in a time crunch. Some will stop at Hobby before going to IAH, so you can end up taking a bit longer than 2 hours to get to IAH