Your question is very vague, however I will attempt to give you a workable answer. To me, it sounds like it could be a few things, but I will say: more often then not when installing a fresh OS, the computer will run slowly. THis happens for a variety of reasons. The main culprit is usually driver related. Please visit your computer manufacturers website for the appropriate drivers for your computer, also make sure to get the appropriate bit (x86 or x64) version driver. If you have an intel processor, you can visit www.support.intel.com then visit tools>updates>auto detector. IMEI, chipset, rapid start, rapid storage drivers make a huge difference in the operational speed of your computer.

It is not uncommon to run into these types of issues related to memory when attempting RAM upgrades. I would suggest always replacing both sticks (if applicable) with the exact same type of RAM, and please make sure the RAM is of the specifications of your particular Mac, PC, etc. I would suggest first trying to clear the NVRAM if you have issues after RAM install. TO do that: shutdown your computer, turn it on and IMMEDIATELY after hearing the hard drive kick, press and hold at the same time, the COMMAND, Option, P, and R keys. Hold the keys until your computer restarts.