I have Samsung Galaxy S GT-i9000, now I want to upgrade its OS from 2.1 to 2.2. However, through Google i have found that many people are facing performance issues after the upgrade. Does anyone have experience with this, and can you tell me how to get 2.2 to perform well?
Performance is generally a very subjective thing. There are apps that will measure specific aspects of performance, but that won't always correlate with what you feel from using the phone day to day.
Generally, one of the reasons for upgrading to Froyo is that it should give you performance benefits, see this previous question on Froyo's performance. In addition to that, Samsung have rolled in some fixes for previous performance problems (if you're using the official Samsung build): amongst other fixes, they've changed things so that the filesystem lag is less of a problem on Froyo; they've changed GPS and aGPS performance to get rid of a lot of those previous problems,;and they've fixed some of the serious problems in their customized apps like the music player.
One thing you may find after any upgrade is that your phone still has outdated data left in its caches and temporary areas that are only relevant to the previous version. These can cause performance problems straight after the upgrade. To get round this, you can either go through all your apps list after the upgrade and clear the caches, or reset your phone to factory settings after the upgrade and then reload all of your data. Be warned that both of these (especially the second option) can and will lose some of your settings and data, though anything saved on your SD card or saved on servers on the web will be safe, so make sure you back up anything on the phone that matters to you before doing this.
Personally, I found my Galaxy S performed much better after the Froyo upgrade. However I did do a factory reset on it (for a different reason related to changing Google accounts over) which will have helped in the short term.