With all advice given: don't worry about speed.. everyone goes their own speed and it's all the right speed. We're all different and I'd focus on what is comfortable and fun for you. The more hou enjoy it, the more you'll do it.

It got a lot to do with leg strength and endurance. You mentioned that you started to do it 1.5 month, that's like still very early into that skill so 11.5kmph is really normal, like REALLY normal, and nothing to stress about it.

Another thing affecting your speed is you still getting a hang of it on when to apply power and when not to. Cycling is a whole new thing you need to get your muscle memory used to. Even if you jog regularly before, the difference in muscle use and timing would impact your cycling speed. Keep riding and have your leg tuned to it, there's no technique or theory to read for this one, you just have to keep riding and you will eventually get it.

Also sometimes city riding isn't always the fastest, it depend on your route. Turn will slow you down, junction will slow you down, pot hole or uneven road will slow you down, headwind will slow you down, ascent will slow you down.

Keep it up! I think you did good for 1.5 month of cycling, riding 33km takes a lot of endurance. I did a sightseeing tour at a neighbouring town recently for 26km and my average speed is 13.7kmph. So i'd say ride in your own pace, it's not a competition.

Who cares. Are you having fun? Because that's all that matters.

Are traffic lights added to your average? On my commute I lose up to a third of time on red traffic lights.

Gearing, tires, and geometry make all the difference in the world.

My Transition Sentinel is only geared for mountain biking. It's a terrible city bike. Tons of shock, high torque gears for steep hills, cannot go very fast. But it's insane when you need to climb or descend mountains. It has knobby, 2.4in tires.

My city bike is an ebike, and even though it's a single speed, it's pretty comfortable going between 10-30mph on that gear alone. The battery allows me to haul lots of groceries or baggage (and climb steep hills), and it's tires are wide enough to not get stuck in tram rails or gaps in the concrete road. I have knobby tires to avoid popping tires, but smoother, thinner tires will be more efficient.

Edit: if you have a shock, try locking it out if it has lockout.

I'd also recommend checking out city bikes, such as road, gravel, and upright bikes. There's an incredible amount of diversity, and a downhill mountain bike is about as far from a road bike as one can get. One can roll over a rock the size of a watermelon, the other can coast for meters off of a pedal stroke. Ebikes also are phenomenal as car replacements (or even just as car offsets), but generally cost $1,500+ with tariffs.

As an (ex currently post child..) mountain biker my threshold for having good endurance or not was being able to ride at pace for hours without sitting down. It takes a lot of fitness to do, but I found when you train like that it comes fairly quickly.

I say that because riding standing I found let's you put more power down and also makes using minimum suspension comfortable as you use your knees instead. You need to find what works re bars and stem though as you can fuck your back up in the wrong position. To ride xc or full sus bikes off the trails I had a second set of wheels with smooth road tyres. Anything but road tyres will dramatically increase your rolling resistance on tarmac.

Are you riding on pavement or trails? Mountainbike tires are generally a lot slower on tarmac, due to the tire pattern, and width, compared to a road tire. Also on tarmac you should increase the pressure in your tire quite a bit.

Don't listen to which speeds people tell you to aim for. Look for improvements in your own rides.

Road surface, hills, wind, tires, style of bike, your height, position on the bike, traffic and many more things have an impact on your speed. Compare yourself with yourself

How long are your breaks? How much time do you spend waiting for traffic lights or letting other vehicles pass? What's your standard speed?

Maybe you could go faster if you did shorter distances. 3 hours is kind of a long time for doing a sport you only picked up 6 weeks ago.