If your computer becoming slow, you are thinking about future proofing it or you have had it for a while and simply thinking about changing it but can not yet afford it I can offer you a range of parts suited for your system and making it fast again.
Most effective ways to upgrade your system would be:
> changing the old conventional Hard Drive (HDD) to Solid State Drive (SSD) would give you the amazing improvement comparable best with getting a very fast new system,
> upgrading memory (RAM), would give you small improvement and visible most in specific professional application,
> upgrading processor (CPU), not often viable in laptops, in desktop computers often require replacing the motherboard and RAM, forced by fast and ever changing technology and least economical,