Because playing game without charging is slower than charging.Is it okay to use a laptop while it is charging? Why or why not?
Let's start with your battery. The life of the LI-ON battery is measured with two metrics. Charge Cycles, and Depth of Charge. You really only have a limited number of charge cycles before the battery really stops holding a charge. However... how many charge cycles you have, depends on how deep the charge is. You have 800-2000 micro charge cycles or so (depends on the quality of the individual cells, how many cells, maH rating of the battery, etc), but you only have 300-600 deep charge cycles. The thing is.... you'll burn through the micro charge cycles faster than you would the deep charge cycles.
This is why LI-ON battery manufacturers recommend that when you use it on the battery, you DO not charge it until it is below 50%, and closer to 25%. Now, you might be thinking that since it seems like the more you charge at one time, the longer the life of the battery... but it doesn't really work that way. If you drained the battery completely every time, the number of charge cycles versus depth of charge doesn't work out in your favor. Optimal charge point is 25% to 50% of total capacity. Newer laptops are even being released with anti-micro charge circuits, where the laptop won't charge the battery unless it has been discharged a minimum amount, and once the battery is "topped off", it won't charge again until it is below that minimum.
Now. Is it OK to use the laptop while it is charging? Absolutely. It is expected. Why? Well, for one thing, most laptops now will underclock the processor, dim the screen, and otherwise cut power usage to give you longer battery time... when you are using it on the battery. So, to get the full performance out of the laptop, either you have to go in and manually change ALL the settings related to power consumption, or you have to plug it into the power adapter to get the full performance. Thus, your experience with games playing slower on battery makes perfect sense.
Now.. if when you say "Because playing game without charging is slower than charging." you actually mean that in BOTH examples (without charging and with charging) the laptop is plugged into the power adapter... then if you haven't been monkeying with your power settings, you might have a problem. Something serious... as in Motherboard Issue. It might also be a matter of using the wrong power adapter. Like... if you purchased a replacement adapter that puts out 65 watts, and the original adapter was 90 watts.
I assumed though, that "without charging" means on battery, and "with charging" means plugged into the power adapter.