Welcome, wady89.

When you say 'decrease' and 'increase', preferably if you can be more specific. Example:
- It takes 0.5s more to accelerate third gear from 80-100kmh.
- Topspeed on the same highway reduced to 110kmh, where it used to be 125kmh.

As for your service routine, have you done the carb and air filter service?
What EO do you use?
What does your spark plug look like when the old one is taken out?

Get the carb cleaned up, and notice how the different engine response when cold vs hot.
It should be difficult to start, sometimes need choke when early in the morning and the bike is cold.
When engine is hot, it should be easy to restart.

Standard air filter, just air gun clean it.
That tyre is fine, IMO.

I don't know anymore, I clean it myself.
Maybe RM15~20.
Best use standard size, 70/90 front, 80/90 rear. 90/80 rear also ok.

Its typical for most bike to decline in perf as time pass, as a std bike user myself, I regularly change my EO every 2000 or 1500 km mileage, you know just for guaranteed maintainance, it also depends on what EO you used as well, ask opinions of what's the best from pomen, I myself use maxoil, most people recommends jtx,shell vsx depends on your budget too though, if significant perf reduction hapens , you might wanna get your bike for some checkup, apart from these advice I got none, maybe you wanna hook up a new cdi

Start with stage 1 upgrade stuff. Spend some time on it.

Stage 1 mod concept covers a lot of stuff, but in essence anything that can be immediately bolted on, or directly swapped out from stock, without additional work to be done to the engine. It is not just about improving performance, but also handling of the vehicle.

- High flow air filter, or removed stock air filter.
- Exhaust
- Lightened wheels, chains and sprockets
- Brake pads, hose
- Engine oil (10W-30)
- Spring, fork oil, monoshock
- Spark plug, plug cable

Stage 2 can be more aggressive of stage 1 mods, require more work on the engines, or adding things:
- Intake+Exhaust port and polish: valve and tappet
- Big bore block + piston kit + carb
- Camshaft
- Crankshaft, jack rod, jack pin etc
- Gearset
- Oil cooler
- Radiator fan
- Weight reduction
- CDI remap

Stage 3 is race preparation, which I don't know a lot about.

Air filter can use UNI, or K&N. Not sure how much. Got other high flow ones.
Brake pads got a lot of brand, Elig, Ferodo are some I know. Making sure the pads and disc do not kena oil is the key.
Engine oil can be RM20-50+. Check page one for recommended bike EO.
Fork oil must make sure both side balance. You can use cap ayam RM3, to Motul.
Monoshock Kayaba is pretty good.
Spark plug NGK G-power RM12. Other like Iridium would be expensive around RM50. NGK plug cable about RM30, red color.

These mods above is to let your bike use better parts, and may require some minor setting adjustments such as pilot screw with the installation of high flow air filters. You can easily spend up to RM300~400, and do not get huge improvements.

Stage 2 will really burn your pocket, and that I have no experience.
Need to ask owners like serigala, thianz, crappy123.

nob1 less power. use ahm m1 or yy pang. if buy second hand just change fiber. sound very power.

It depends what you want. You asked for minor mod...

if got money and spare bike. straight modi 62mm with 19-22 valves jack 4mm pin nsr carb. sure satisfied.
can ride daily also no problem but makan petrol. depending on how you mechanic setting also.

max at 120kmph. moto meter.

Stock condition mia LC can be like that, especially if the carburator have never been serviced.

Check out here for a list of users who tracks their bike fuel consumption : https://www.fuelly.com/motorcycle/yamaha/lc135
As you can see, average users are getting 44-46 km per liter, so at full tank of 4 liter, that's about what you can hope to achieve.

Normally, I'm guessing, you would fill about RM7, which is 3.33 liter, multiply by 45km per liter, you get 150km per tank under normal riding condition.

After you service the carb, then track your mileage again.
The main tip to save petrol is all in the throttle control:
- Do not rev your bike at traffic light. Especially for VM carbs (mikuni round slide, all version except ES v2) because there is accelerator pump.
- Check out this video, it shows how the accelerator pump injects petrol stream into the carb as the throttle is pulled.
- Go easy on the throttle when twisting. The slower you twist it, the lesser the accelerator pump injects petrol.


Other than that, you can try the tips below:
Pilot jet screw:
- For LC, this is a pilot fuel screw, so, the more turn out, the richer the pilot fuel circuit is.
- During carb setting, if it takes less than 1 5/8 turn to get a smooth idle for pilot screw, change to a smaller pilot jet.
- The ideal pilot jet and screw setting is, at around 1 5/8 turn of pilot screw, the bike will need a choke to start when cold, but once warmed up, off idle throttle response is good.

The above is step one, to improve your carb setting... if you can do it, let me know, then I will share some more.
It will take you a few days to figure out if the setting is right, because you need to let the bike sit cold overnight, and try to start it the next morning.