This issue does not affect me directly, but my friend (does not have a forum account) has been experiencing this ever since he got a new mouse and keyboard.
He says that when tabbing back into Blockland, whether using Alt or just Esc-clicking back in, it freezes up the game (and only this game) for a good three minutes. He's tried clean-installing to Windows 10 from 7 and it has not fixed it.
His specs:
Windows 10 pro 64-bit (10.0, Build 14393
Intel(R) Core(TM) i7-3770 CPU @ 3.40GHz (8CPUs), ~3.4GHz
12288MB RAM
NVIDIA GeForce GTX 1050 Ti