TCL TK Progress Bar for large file transfer: too much time - copy

Question

I suggested this solution to a colleague that needed to implement a progress bar for file copy. Here's the code, basically a modified version of another example I found:
proc CopyMore {in out chunk bytes {error {}}} {
global total done transferredBytes totbytes
incr total $bytes
incr transferredBytes $bytes
if {([string length $error] != 0) || [eof $in]} {
set perc [expr int($transferredBytes*100/$totbytes)]
updprog $perc
puts "$transferredBytes $totbytes"
puts $total
set done $total
close $in
close $out
} else {
set perc [expr int($transferredBytes*100/$totbytes)]
updprog $perc
puts "$transferredBytes $totbytes"
fcopy $in $out -command [list CopyMore $in $out $chunk] -size $chunk
}
}
proc Copy {in out chunk} {
global total done
set total 0
set start [clock seconds]
fcopy $in $out -command [list CopyMore $in $out $chunk] -size $chunk
vwait done
set end [clock seconds]
return [expr $end-$start]
}
proc CopyFile {source dest} {
set chunk [expr 1024*1024*18]
set in [open $source]
set out [open $dest w]
fconfigure $in -translation binary
fconfigure $out -translation binary
retur[Copy $in $out $chunk]
}
The CopyFile function is called for every file I need to copy. The fconfigure $in -translation binary seems needed here, because I'm handling binary files (already tried to remove this instruction and just a few bytes were actually copied).
I supposed that this copy was designed for large files, but I noticed that the copy process is extremely slower compared to the previous solution (without chunks, progressbar, etc). Here some numbers to copy about 1.3 GB:
Method
Time (secs)
Previous solution
9
Chunk 256K
178
Chunk 1M
143
Chunk 18M
127
Chunk 100M
122
Chunk 300M
121
So it seems that increasing the chunk size slightly improves the performance but all these results are very far from the previous solution.
I can't explain such a difference. Do you guys have the same issue?
Thanks in advance for your help.
UPDATE! I added the following instructions in the CopyFile function:
fconfigure $in -buffersize 1000000
fconfigure $out -buffersize 1000000
and now this new copy tecnhique is about 20% slower than the previous one.
Do you guys have any suggestion to reduce more such time difference?