Help my MacBook becomes hot and then becomes slow

Computers need to be cleaned inside every now and then to get back the correct air flow to allow proper cooling. Being a 7 years old Mac it would need to remove fans, logic board and heatsink to clean them up..heatsink might be partially or very clogged by dust. Thermal paste is probably dry after all this time and needs replaced. Summertime temperatures just make things worse. Overheating may damage logic board beyond repair sometimes. I would suggest before anything else to take up this task yourself if you feel like it or just pay a pro to do it for you. In case you decide to do it yourself this guide will help you in the task step by step MacBook Pro 13" Unibody Late 2011の基板の交換