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
