Lots of things to evaluate but if you’re relatively new try stuff like this.
Do you feel stretched out? Cramped? Do your shoulders or neck hurt after a while?
Too tall on the bike (like it feels high off the ground or tippy) or is the seat post way far out for your legs to be comfy?
Side note: it’s very easy to run your saddle too low, make sure you’re close to correct leg extension. You can google that one....
Some stuff like pedal efficiency is hard to quantify but are you struggling to keep up with others? Feel like it’s dragging uphill?
Do you feel confident going downhill or nervous?
Are you able to put the bike where you want to in turns or can’t keep a line?
I’m trying to think of things that come down to feel but at least you can compare to a different platform.
Modern bikes are pretty good. It might just come down to what feels right under you. Pick a category and price range and try that.
If there’s a demo available for something you haven’t thought of maybe try that too just to see what the difference is. Maybe you’ll love a XC or enduro style bike more.
Remember this is all for fun!
