A few of us who edit the work of others in a small office have been using the desktop Windows version for a year and a half. We have an unusual writing style that wouldn’t be like anyone else’s. We have added many words to ignore or prohibit.
I have created a couple of rules with the Rule Editor just to see whether I was able to do it. The next step would be to create more rules. If we add 5,000 rules, would the desktop Windows version crash or become too slow? Our PCs are typical PCs–not underpowered but not state-of-the-art. Some of our documents are hundreds of pages long and the desktop Windows version has been handling them well.
If you’re talking about English, it already has about 2100 rules. If you add another 2100 rules, you can assume that performance drops, but not quite to the half, because there’s more work done in LT than just matching rules. So adding 5000 rules could make checking maybe 2-3 slower, but it shouldn’t crash LT.
There are also some tools that may help, e.g. if you just need to add couple of thousands of prohibited words with potential replacements you could use simple replace rule: put your words in a format like this: languagetool/replace.txt at master · languagetool-org/languagetool · GitHub
It’s much easier to maintain and it would not affect performance much.