LSTM Querrry....?

I want to use LSTM for mnsit classification , i have created two cell arrays for input data and labelled data both 0f 5000x1 , X nad y respectively , X have each cell of 400 x 1, which is the 20x20 image ,Is it the right approach??? but the problem is that i have to set T = 5000 and its taking too long to train the network..
Any suggestions 