This is doing single threaded search&replace in linear time (~1 second per ⅓ million @ 2.60GHz). Those numbers already include all the I/O to be done for the replacement.
cargo build && ./target/debug/fastsearchandreplace < wordlist.txt | wc -l Compiling fastsearchandreplace v0.1.0 (file://[…])
Finished dev [unoptimized + debuginfo] target(s) in 11.9 secs
time: 14.926014173
5000000
48185 keywords.txt
5000000 wordlist.txt
Each line in wordlist.txt is a single word. Each line in keywords.txt are two tab-delimited words, first the key, second the replacement. The replacement string is of random length and therefore not suitable for in-place replacement. Output with replacement done is written to stdout, and a time written to stderr.
You can generate new wordlists using:
# this will generate 5000000 for the wordlist and a hundredth of that for the
# keywords (note: keyword is dedupped)
./wordgen.py > wordlist.txt 2> keywords.txt