Is there a reason why should I rush to rescue Imoen? I was thinking of doing most quests in Chapter 2.

Rush to rescue Imoen if you want her in your party (so she won't be levels behind the rest of the game), but otherwise you can take your time.

Hm I see. How many levels behind?
Actually I want Imoen. Are there some quest if I rush to rescue Imoen that later I could not start-finish them.
I hate to think I missed something. ;)

I think you won't miss any

The quests which can be done in chapter 2 and 3 can also be done in chapter6.

IIRC, Imoen's around level 11 when you rescue her.

I usually do anything relating to the graveyard area in chapters 2 and 3, that way when I return, I can avoid it until I'm ready to finish the game.

Imoen will have 400,000 XP as mage when you get her back.

I think it is higher than that, IIRC (most probabaly I don't) she had over a million when I rescued her.

I did everything in Athkatla besides the Rogue Stone Doorway and Kangaxx in my chapers 2 and 3, than by the time I was out of the underdark, Imoen was about half a level behind Edwin, who had been with me for most of the game. So I don't think she ends up all that far behind.

When your average party class level is very high, you can get a version with 1,200,000 XP. (Don't know if this requires ToB)

If you have Nalia in your party it's nearly impossible, her low levels will lower the average party class level too much. My party had done all quests in chapter 2-3, but she still had only 400,000 XP. When I dismissed Nalia before I got the 1,200,000 XP version.

Indeed, there are exactly two versions of Imoen that you can meet at Spellhold. One with 400K XP (level 7/11) and one with 1200K XP (level 7/13). But generally when I do everything before Spellhold, even without any quest mods, everyone's at least 15th level.