calculating is too slow in my excel sheet

[belal]

first of all, i have to admit that i learn many issues in excel from this site so i have to say thanks a lot..keep it up

Second, i have major problem in my excel which can be concluded in slow calculating ( on other words, every time i try to update or modify in sheet excek took long time in calculating like this (calculating : 4 processor ..19% ) ...my excel sheet include many functions like index & Sumif & vlookup & match & pivot table & If error and for sure all this function may be mixed in one cell...so what shall i do ? i search a lot and made several tries as i read it on web sites like give excel sheet high priorities and make calculation manual and this option i don't like it as it will not help me in my work

my excel sheet have several tabs and it's size around 14.5Mega and my laptop specification is below :
Model :hewlett-packrad HP Probook 4540s
Processor : Core I5-3210M CPU @2.5 GHz
Ram :4 Giga ( 2.63 GB useable )
System type : 32- bit
Microsoft office professional plus 2013

 

[Aladin Akyurek]

Correction to 3...

=IF(VLOOKUP($A16,CstMonths,1,1)=$A16,VLOOKUP($A16,CstMonths,MATCH(C$2,INDEX(CstMonths,1,0),0),1),0)