High fuel consumption and slow pick-up can be because of several problems. I have divided the problem and solution for you into two categories.
Basic problem is.
1- Check your airfilter. Make sure you use Guard airfilter.
2- Use carborator cleaner spray and spray into the intake of airfilter box. I recomment you 7ct carborator cleaner spray.
3- Use Denso plugs K16R-U. Gapping should be between 1.0-0.8mm. These plugs will cost you around 175rs each.
4- Remove your CAT converter. Vitz has got two CAT converter one is underneath the headers. And the other one is like a Dholki. After removing the dholki CAT converter you can install another dholki or just a simple pipe. I will recommend you a dholki otherwise your car will produce some stranger sound.
5- Check your oil and oil filter make sure you use syenthic oil. I will recommend you Zic A+ or mobilone. Change your oil after 5000km this is for Zic A+ or if you use change it after every 8000km.
If your problem still persist. You have to go through something major.
1- Get your fuel injector serviced from a good mechanic. It will cost around 1500-2000rs.
2- Check your fuel filter if it is chocked get it replaced with the orignal one. After every 40000Km you need to change your fuel filter.
3- Check your fuel pump is it working perfectly. If it is week get it replaced with the genuine.
4- Check your transmission oil. If it is below the level get it changed with ATF. Don't use ordinary oil. This is also changed after every 40000Km.
5- Lastly your MAF sensor could be week. Get it serviced with a electrical circuit cleaner, please don't use WD-40.
Use orignal parts.