PHPStorm constantly lagging when I press CTRL S

Everytime I (instinctively) press CTRL S the IDE just becomes unresponsive for around 2 minutes, this is unbearable, what is the cause of this, how can I fix it?

Task manager are showing low use of CPU, with free ram and 100% disk, but top user (of disk) is Dropbox.

This happens only on Windows 10 desktop and not on my Ubuntu laptop; My version is 2017.3.2

Please help. Thanks!