gpu slower than cpu for neural networks in matlab -

i have following code:

tic; h = rand(100, 1000);     f = rand(1, 1000);     net = newff(h, f, [30, 10], { 'tansig' 'tansig'}, 'traingdx', 'learngdm', 'mse');      net.trainparam.epochs = 400;     net.performparam.regularization = 0.05;      net.divideparam.trainratio = 1;     net.divideparam.valratio = 0;     net.divideparam.testratio = 0;      net.trainparam.showwindow = 0;     net.trainparam.showcommandline = 0;      % net = train(net, h, f, 'usegpu', 'yes', 'showresources', 'yes'); % line 1     net = train(net, h, f, 'showresources', 'yes'); % line 2  toc; 

with line 2 uncommented

computing resources: gpu device #1, geforce 800m  elapsed time 5.084222 seconds. 

and line 1 uncommented

computing resources: mex2  elapsed time 1.870803 seconds. 

why gpu slower cpu? gpu properties:

cudadevice properties:                    name: 'geforce 800m'                  index: 1      computecapability: '2.1'         supportsdouble: 1          driverversion: 6         toolkitversion: 5     maxthreadsperblock: 1024       maxshmemperblock: 49152     maxthreadblocksize: [1024 1024 64]            maxgridsize: [65535 65535 65535]              simdwidth: 32            totalmemory: 2.1475e+09             freememory: 1.9886e+09    multiprocessorcount: 1           clockratekhz: 1475000            computemode: 'default'   gpuoverlapstransfers: 1 kernelexecutiontimeout: 1       canmaphostmemory: 1        devicesupported: 1         deviceselected: 1`