What is use speed?

[Shark Lord]

I'm trying to build my doc, and i have the field surgeon card. It says -50% use speed, now my question is what does this affect? Is it only opening chests and doors, or does this affect healing and revive speed? I have googled this and have found almost little to nothing on what use speed actually is, or what it effects. Anybody with any information please enlighten me. Cheers

[GreenShades]

Most action which has a progress bar such as healing and using the howitzer in the Finale of Act 1