If you are seeing a slowdown during the training, you might (accidentally) store some tensors, which are still attached to the computation graph e.g. in a list. This would however also increase the memory usage. If that’s not the case, your system might reduce its clock due to e.g. overheating, so you could also check the sensors in your system.
PS: you can post code snippets by wrapping them into three backticks ```, which would make debugging easier, as your code is currently hard to read