Odd timing behaviour in reading a file

Hi all,  please don't ask me why I tried this but.......

I have observed some odd behaviour in the time taken to read a file. I 
tried searching the archives without much success, but that could be me.

The first time I read a (60Mb) CSV file, takes a certain amount of time. 
The second time takes appreciably longer and the third and subsequent 
times very much shorter times. See below,

$ R2.1.1

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 > system.time(temp <- 
read.csv("Mapping50K_Xba240_annot.csv",header=TRUE, as.is=TRUE))
[1] 32.55  0.30 33.46  0.00  0.00
 > system.time(temp <- 
read.csv("Mapping50K_Xba240_annot.csv",header=TRUE, as.is=TRUE))
[1] 45.32  0.24 45.72  0.00  0.00
 > system.time(temp <- 
read.csv("Mapping50K_Xba240_annot.csv",header=TRUE, as.is=TRUE))
[1] 11.73  0.17 11.94  0.00  0.00
 > system.time(temp <- 
read.csv("Mapping50K_Xba240_annot.csv",header=TRUE, as.is=TRUE))
[1] 8.58 0.28 8.96 0.00 0.00
 > system.time(temp <- 
read.csv("Mapping50K_Xba240_annot.csv",header=TRUE, as.is=TRUE))
[1] 8.80 0.16 9.02 0.00 0.00

This is a relatively quiet opteron running redhat linux and using R2.1.1
The same pattern is repeatable, and occurs in R2.0.1 and on a Dell 
laptop running Windows XP.

I guess it is probably something to do with the garbage collector? Can 
anyone explain further? Particularly the first increase.

Thanks....