Thanks for the report, you found a real one and it's now fixed for the next update. Good news first: it isn't the save system, and the save fix from the last update is working, so nothing you buy is being lost.
What actually happens is that a Max purchase buys levels one at a time, and the game was redrawing the entire upgrade list after every single level. The last update also made milestone Scrap scale up at higher Voltages, so a Max purchase can now be thousands of levels in one tap. Thousands of levels times a full redraw each is what locks up the tab.
The fix makes the screen redraw once per purchase instead of once per level, which brings Max back to well under a second. Until the update lands, the x50 multiplier works without the freeze.