It will not slow down the computer.
But that depends what is compressed and what is not.
For example if you have compressed data of some program(s) you frequently use then that data will need to be uncompressed before you will be able to use that specific program(s) again and that uncompressing time will be the time when you wait and "think" that you computer works slow.
So yes compresing does free up some space (depending what kind of data is compressed),but it then takes some time to uncompress that data if you are going to use it again and during the uncompress process you need to wait.
Also NEVER compress the ENTIRE hard disk drive because that is not the smartest idea because every time when you use the computer,Windows AND you obviously use some data in order to operate properly.Yes...it will work,but the computer will be slower since the data must be uncompressed before it is able to be used by either you or Windows or both.
But like I said...it really depends what data is compressed and what data is not compressed.
That is why you should always compress only the data which you do not use frequently instead of compressing ALL THE DATA on the entire hard disk drive or solid state drive or whatever type of media you use.
So by default only the data in the root directory is compressed automatically.You CAN select to compress ALL the subfolders on the entire drive,but as I said before...it is not the smartest idea.
Anyway to uncompress everything back,just remove the check mark from the check box option called COMPRESS DRIVE TO SAVE DISK SPACE and then click the APPLY button.The process of uncompressing should start so let it finish and tell it to IGNORE ALL when it comes to encountering opened files that it cannot modify.
Cheers!