Are you using a laptop? It could be anything from Wifi signal issues to your neighbor downloading porn on your network to some other networks creating interference...
Edit: if your ping is fine then it is probably just your computer not being able to handle GTA (lol)
Go into settings and use the lowest options possible for everything, lower drawing distance and report back
Disable your real-time anti-virus while playing, that can also slow you down if you use Avast or Avira since they are resource heavy programs.
Also, make sure you aren't playing with /fpslimit 20 set, it happens to me that I hit the bind by accident and then I rejoin with a laggy SAMP until I remember to do /fpslimit 80
