how slow are modelsim free licences?
ModelSim PE Student Edition limits the user to 10,000 lines of executable code. The performance of your code up to this capacity is 30% that of the full version (the full version is 3.3x faster). The student edition can still run code past the 10,000 line capacity, but it slows the performance down dramatically to 1% of the full version (the full version is 100x faster after this point).

I got this info from the 'Performance' section of Mentor's ModelSim PE Student Edition webpage:

https://www.mentor.com/company/higher_ed/modelsim-student-edition