I have a lenovo a7000 plus and its heat after 15 minutes of playing games like subway then gets really slow?

Ram 2 gb
Processor 1.7
GPU arm Mali 760
The game starts like in 60 fps the gets lower 20

This is called thermal throttling. It's when a CPU or GPU slows itself down to prevent itself from overheating.

Since it's a phone and you can't install any external cooling, your best bet is probably an RMA.

Hope this helps.