Multimode interference (MMI) devices operating at high data rates are important for integrated optics and optical networks. Their 1 × N splitting provides a basic functionality in these applications. Ultra-high speed data transmission at 40Gb/s per channel with a total bandwidth of 320Gb/s for all 8 output ports is demonstrated for the first time on a 1 × 8 photo-definable polymer-based MMI power splitter. The transmission integrity is confirmed by the bit-error-rate (BER) testing. To further predict the speed limitations of MMI devices, ultra-short pulse response of these devices is quantified. For example, for 20 fs Gaussian input pulses into a 1 × 8 polymer-based MMI splitter, the output pulses are severely degraded in coupling efficiency (47%) and completely broken up in time and in space primarily due to inter-modal and intra-modal (waveguide) dispersions.