Optimize bash script that concatenates output
do ... done is a compound command; every subcommand shares the file descriptors; so teeing the the loop has the same effect as teeing each subcommand.
Two subsequent invocations of echo can be combined together.
cat $file is a dreaded UUOC.
A basename invocation can be avoided by changing directory to $FOLDER.
ls is absolutely unnecessary. The shell already globbed the *.csv.


Summing up,

    chdir "$FOLDER"
    for file in *.csv; do
        echo -n "$file" "|" | sed -r ...
        head -1 "$file"
    done | tee $LOGFILE


does the same job.