My Passport Ultra 2 TB getting extremely slow and sometime non responsive

I have been using the Drive for past 8 months it was working fine but suddenly I got very very slow and sometimes it gets completely un responsive. When I plugged it in it takes some times to show and i can see my folders etc. but when I start the copy process it starts in Kb and some time in will jump to MB for few secs and again falls to Kb and then stops. I have got important files and I dont want to replace it . I used different usb 3 cables but it is same. I opened the circuit board the circuit board connectors to hard drive had some sort of dark substance I cleaned it up but still its same. I suspect the issue is with the circuit board. If I find a new circuit board will it work with this drive. I have removed the password from the drive. The drive is not making unusual noices only when it is unresponsive for long time I hear the Tick of the head parking.

I have also tried to use the drive on work computer its the same Issue

Replacing the circuit board may result in a hard drive you can no longer access due to differences in firmware and production revisions. I would recommend contacting a professional data recovery service company in order to extract the data instead of attempting to repair the unit.