Not done one of these in a while so here goes: if you think your bicycle is a bit too tough to ride as per effort, here's some pointers. This is obviously aimed at people new to bicycling to give you some idea of what's worth to do and what not.
The "average" human (heavy air quotes on this one due to the range of humans) can continously put out 100 Watts on a bicycle for an hour with generally no problems to give you some scale here.
I'm assuming here you don't actually want to buy a new bike.
Free or Nearly Free
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Put air in your tyres. Range of TPI or Bar is printed on the sidewall somewhere, generally the higher the easier to ride but it does get bumpier. Don't eyeball it except on thick MTB tyres you don't use for any kind of sport, get something with a manometer. This is basically impossible to calculate in watts on account of it changes heavily with road surface, i.e. lower tyre pressure might be faster on shitty roads.
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clean and oil your chain. Like just wipe it with a rag and then use some bicycle chain lubricant (maybe 8 eurodollars a bottle, so about 0,02 per oil application) - this can mean a difference of 10 - 20 Watts
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Change your position on the bike. Aero is big for bicycles, if you can assume a more streamlined position there might be a another 10 - 20 Watts in there. Just change your bars, it's a few screws and maybe 10 eurodollars for a new one that gets you a bit lower
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dial in your saddle height. Just play around with it, just make sure your leg is never fully extended or you might wreck your joints. This can be another 10 Watts (more if your current position is just absolute dogshit)
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if you have a a 2x or 3x in the front, maybe take a look at what gears you're using and how hard they crosschain. The closer you get to straight the more efficient it is. Watt savings can be up to 5 Watt with few differences in how it rides since 2x or 3x often have the same gear ratios for multiple gears.
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Unmount shit you don't need. This is kind of case by case and might not be worth the hassle if you do use it occasionally, but between carrying an air pump, a rear rack that sees usage once a year, a fixing kit that sees useage twice a year you may be getting like half a kilo of weight off there for no cost.
Worth it for some money
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New Tyres with lower rolling resistance. You can find good tyres, where I am, for ~30 eurodollars if you can get them on sale. Depending on what you have now this can be a 10 to 30 Watt difference.
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Wheel replacements. This can be a bit of work scouring your local online used market, but you can semi regularly find generally good bikes that have quite good rims but are otherwise in a state of horrid disrepair for quite cheap. Most wheel bearings for rims are closed, which means you really have to try to fuck them up bad. Could be another 10 Watts for 50 eurodollars or so.
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getting new pedals - a lot of mosey on around town bicycles seem to come up with plastic pedals with rubber grippads that have last had friction two decades ago. Change these out for some basic metal pedals with pins - it feels way better to ride and due to less of your motion going into slipsliding across the pedals you could get around 5 Watts.
Do not
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Swapping parts out for lightweight stuff. If it saves actual weight it costs a gajillion dollars, otherwise losing a kilogram of your weight (unless medically inadvisable) is probably cheaper and worth more
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Drillium, for obvious reasons
I'm gonna be a big nerd and politely give advice that might conflict with yours.
Tires PSI is the easiest and cheapest way to improve comfort. Do NOT aim for what is printed on the sidewall of the tire. That's often the MAX PSI and aside from being uncomfortable, it is on the edge of where your tire might wear prematurely or worse. Instead use an online calculator like the Silca one where you put in your weight and the tire width and butyl tubes and cat 1 gravel. Your tire width is printed on the sidewall. If it's around 32 mm you will end up between 30-50 psi depending on how much you weigh. Lower PSI = a comfy ride so long as you're within a safe and controlled range. The tire will absorb vibrations.
For position and overall bike ergonomics, it depends on your type of bike, which then dictates the kind of riding you'll want to do. An aggressive road bike will be fairly uncomfortable if you ride it too upright. A relaxed city bike won't really reward you the same way when you lean forward (though you will get more athletic ergonomics usually). With the road bike it's a fitness device, you're meant to be pedaling fairly hard 90% of the time you're on it and so if you're just tooling around all the time with little pedaling you'll run into issues like putting too much weight on your hands (hard pedaling takes weight off the hands). I recommend thinking about which kind of bike matches your goals with the bike, if you're buying one.
Buy used. You can pay 1/5 the price of a new bike by getting a very nice 5 year old bike that is functionally the same after some inexpensive changes (new cassette, chainrings, chain). Sometimes those things are in perfectly good shape, too.
Seat height is a very good thing to tweak. Most people put their seats too high. Drop it until you think it will bother your knees and then go up a bit.
The best and least expensive upgrades are contact points. Saddle, handlebar tape/grips, pedals. Saddle is most important. Don't get something too padded. Just a little bit of padding, focus on sitting on it stably and finding what the right hip rolling (forward/back) is correct for your bicycle's geometry. Shops will sometimes let you try out saddles. Sit bone measurement means basically nothing. Just try saddles and get a feel for it. Don't spend too much at first. A used wtb is not a bad place to start. Maybe $20-30? For grips and pedals I recommend cheaping out and going AliExpress. Exact ones depends on your bike and style but I spent $12 on my favorite mtb style flat pedals.
Adjust saddle tilt. The thing you want to eliminate is any numbness. Legs or groin etc. Numbness is always wrong. Adjust your seat / position or get a different seat until it's gone. Saddles with a cutout are almost always better for this.
The Best thing you can do is ride. Get your butt used to riding and a normal low-padding saddle will be comfortable. Develop core strength from riding and you'll have less pressure on your hands and can adopt a more aggressive position if that's what your bike wants you to adopt.
Tires are one of the best "upgrades" you can make and choice depends on your bike. Wider is more comfortable but you have to make sure your frame has clearance. Low rolling resistance is nice but once you get to a certain range other things also matter. Local bike shop used tires are often an amazing discount. Almost everyone should ride slicks or semi slicks unless they're in the mountains or already know what they need. If you're on road you cannot beat continental gp5000 (though they are not cheap, everyone will tell you to get them) but I honestly recommend the ultra sport iiis ($20-30 per pair sometimes!) because they're about 90% as good. Gravel tires are great, too.
Don't sleep on fenders. Like the win wings, they're fantastic clip ons. Keep yourself cleaner, ride in the rain.
A light and reflective clothing etc might save your life.
Do tyres where you are not give you a range? Genuine question because I was unaware that there are ones that don't do that excluding maybe kids bikes that nobody cares about or whatever
Most tires have a max PSI, not a range. Max PSI is a manufacturer communicating information about when the tire might blowout, you want to stay below that PSI to avoid catastrophe. Too low of a PSI is more variable and left up to the user. You can usually kind of tell, though - it rides worse and starts "burping" from the rim, eventually causing a snakebite puncture for a tube or losing pressure and spraying everywhere if tubeless.