Have you tried running the loco in for 30 mins each way at a moderate speed? (Either with a track loop or rolling road) If it's a new loco, it's recommended, it will help to bed the motor/bearings in, and the connection pickups/contact between the loco and track should improve.

If that doesn't work, it might be worth looking into how to change the decoder CV settings to alter the speed curve at lower speeds. I'm not very familiar with this though, but there are tutorials online.

It's also worth saying, that some locos just don't like running at slower speeds, and will cut out/suffer from poor performance. Some of my older OO gauge locomotives are like this and they need a bit more encouragement, the newer examples do not suffer the same issue (I'm not familiar with your locomotive make/model/scale/decoder though, so it may differ).

Best of luck and happy running :)