The PST questions are not difficult, but finishing on time is. Luckily, there is plenty of material on the web to prep for free (including on the McK website). PrepLounge also has some excellent material, which will take you a long way.
Don't forget also to prepare the quick math calculations; caseinterviewmath.com in particular will rank you by accuracy and speed.
Check PL's material there:
- https://www.preplounge.com/en/tests/mckinsey-pst
- https://www.preplounge.com/en/tests/library
Hope this helps -