It seems like everyone is trying to outdo others on 'water saving' devices, including toilet flappers. That often means a valve that doesn't let enough water through to do a decent flush.
Many of the manufacturers offer a valve with an adjustable fluch. They are usually at the higher end of the price range. That is what you will want to look for, one with adjustable timing. Set it at the maximum time range and see if it meets your needs. If not, take it back to the store. If it gives an adequate flush you can always dial it back to save some water. With a lot of valves, when you have solid waste to flush, you may have to hold the handle down for a little while to get long enough water flow. But for just liquid waste, a simple push down and let go should always work.