Hi.Yeah,downgrade the RAM to 8Gbs & UPfrade the CPU to thefastest Your motherboard can take.
Problem 1 ) Celeron ANYTHING is a bottom-of-the-line budget processor ...
Problem 2 ) Having a huge RAM bank for the CPU to access, organize, & delegate to multiple processes is even going to slow down a decent fast CPU, combined with problem 1 You're looking at an electric brick instead of a computer.
Also, You say the system recognizes the RAM as 16Gbs. This means that the Windows version You have installed is a 64bit version, which will always run as crap on a Celeron CPU.
If You drop the RAM to 4Gbs & wipe the 64bit Windows for a 32bit version You'llnotice a serious improvement on the Celeron,but if You upgrade the CPU on the 16Gbs with the existing 64bit Windows You'll notice the best improvements.
Although I stillrecommend the CPU upgrade AND downgrading to 8Gbs of RAM.