Very first thing, ensure the water supply shutoff knobs are fully open. Could be the electric valve(s) inside that are stuck mostly closed. They are not terribly complicated and a fairly easy replacement if it comes to that. When you remove the supply hoses from the washer, you will see screens on the washer side, that may be clogged with sediment or mineral buildup. You can see the screens here: https://i.imgur.com/5cGwJj6.jpg

Finally, it may just be how it is. I replaced my washer a few months ago, and the old one worked, but had always filled very slowly.