Vim surprisingly slow?

Taking on the challenge listed here

https://dev.to/jorinvo/csv-challenge-1al

it provides a .json file link to mung into date-named files (never
mind that these happen to all be the same date), I came up with this
vim solution:

:g/.*name":"\([^"]*\)".*card":"\([^"]*\)".*/let
s=substitute(getline('.'), '.*stamp":"\(\d\+\)-\(\d\+\)-\(\d\+\).*',
'\1\2\3','').'.txt'|s//\1,\2/|exec ".w!>>".s

which works. But for some reason, it is *painfully* slow on my
machine. The basic intent is that it identifies rows with a (fake)
credit-card number, snapshots the YYYYMMMDD.txt filename, then turns
the row into a CSV entry ("name, CC"), and writes (appending) that one
line into the output filename.

The whole thing should happen in a blink. Maybe a second at worst.
But this was walk-away-do-something-and-come-back slow.

If I change the "exec" to an "echo", it's as fast as I expect.

Any idea what might be making the exec so slow?

-tim




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