I'm a reliability engineer, and I worked at Apple for 16 years, and I might be able to provide some insight.
"Does it effect battery life?"
If you're only running on battery power, gaming will discharge the battery quicker than normal and, therefore you'll have to recharge more frequently. The battery has a limited number of charge/recharge cycles. If you game with the power adapter plugged in, the battery life will not be affected.
"The overall speed of the machine overtime?"
Perhaps very marginally. Heat will cause internal change to molecular structures in chips and electronic components, and they are subject to the electronic equivalent of mechanical wear. Taking a brand new computer off the shelf, and comparing it to an identical computer 5 years old, there may be a 1-2% difference in overall speed.
The best advice I can give you is to use yourportable computer while it's on a flat, hard surface - a desk , for example. Using it on a think carpet or on a bed can cause the internal temperatures to rise. Another hint is to use a vacuum cleaner every month at the rear and side vents, to clean out the desk. Under no circumstances should you use compressed air. The air can cause fans to break, and push dust and debris into the computer.
Enjoy.