Medical Image Segmentation is a process of segmenting abnormalities from normal tissues. Due to the increasing growth of deep learning, there are various deep network models used to segment 3D medical images. Recently, U-Net and V-Net are used to segment 3D medical images. But these networks suffer from high computation burden. The objective of this paper is minimizing the computation time by reducing the input data. Initially, the 3D slices are reduced by taking the average of few slices (Inter-slice reduction). Then, only the tumor area is segmented using detection window (Intra-slice reduction). The reduced 3D Magnetic Resonance Imaging (MRI) data was fed as input to UNet with Long Short Term Memory (LSTM) layers for segmentation and classification. BRATS 2017 and BRATS 2018 are tested by proposed method of dataset. It achieves 96.24% accuracy, 90.84% Dice Score Coefficient (DSC) on BRATS 2017 dataset and 92% accuracy and 88.88% DSC on BRATS 2018 dataset in 12 and 10 seconds respectively. The proposed method is compared with some recent methods. It achieved reasonable gain in computation time with negligible loss in other metrics.