If a reinstallation of the OS doesn't fix the slowness, there might still be a hardware issue. My sister once spilled a few drops of water on her laptop after which it was also incredibly slow. Had to replace the motherboard in the end to fix the issue.
Also turn off laptop. Take out battery. Open RAM cover. Make sure RAM is seated correctly. (while it's open make sure it's all the RAM that the laptop came with.
If the HDD was not replaced then it could be that the HDD is damaged.
If they replaced the HDD then it could be that ''they copied the old drive'' & in the process some files got corrupted.
To get a fairly good idea of if the HDD is dodgy, check its SMART status. Google 'HDDScan' and download and run it. If any of the SMART values are yellow or green, you could have a problem. Or it could be the m/board, software after cloning if the HDD was replaced as Venomous said. Could be a number of things. If I were you I would check the HDD, reinstall the OS and take it from there.