There should not be any issue with the large topics if your wpForo database tables installed with InnoDB engine. After the migration i recommend optimize and repair o tables using phpMyAdmin Tools.

However, if you have an option to split topics it would be better. Even if there is a small impact of the performance you'll  avoid it.

Also, please remember that the Extended and Simplified forum layouts better perform topics with dozens of thousands of replies. If you use the Threaded layout, don't forget use the button in Dashboard > Forums > Dashboard admin page after the migration.

I'm moving topics from my old forum to wpForo installation. Some tiopics have a lot of messages - up to 10 000. Old forum is punBB, it is simple but rather fast, it has no troubles with large discussions.

Well, I need and advice from your developers: should I split large discussions to a few topics for better perfomance - for example, each topic up to 2000 posts ("archive" topics and actual discussion)? Or there is no difference for wpForo how many messages in topic?