I can not keep up on cross country group rides, and that always ends up leaving me feeling badly so I avoid them. Downhill I can keep up and that is where I have learned a few things.
The #1 thing that made me the faster was good brakes. Brakes that I could lock up using only one finger. Getting the brake levers so my wrists don’t need to rotate up or down up grab on. Downhill I don’t shift alot but having your shifters mounted so you don’t have those wrist movements will help too. I would avoid the shifters where you twist the grips to change gears.
When I do ride cross country the best thing I found by far was a dropper seat post. I could get low and my weight back when the ride gets technical with roots and rocks. When climbing or just grinding out the miles I can get my butt up and extend my legs. They are not cheap, $300-$400 but I bought one and kept riding my klunker.
Find you flow, and keep going.