It can be many reasons that you have lags with big packets here some reasons what can be true:
1. Your hardware is not the the best to play Gmod and bigpackets (try to upgrade your prozessor and buy more RAM thats the heart to play laggless and with no crashes)
2.You have maybe wrong settings or settings that are useless (ingame settings+ bigpackets settings) :/
3.You should turn off many programs while your playing your PC/Notebook would be thankful

i hope i helped you a bit more ^^