Installation of RAM making MBP slow
There can be several reasons.

Firsts, new RAM can be slower than your original RAM. Compare original RAM specification and the one you installed. For Macs manufacturers usually have specially tested and tuned RAM modules. Anything cheap will actually decrease performance.

Secondly, Mac allocates as much swap as you have RAM. Earlier your Mac allocated 4GB at start, now it has to allocate 16GB. If you do not have SSD, this will be much slower because the swap file has to be written to disk at start.