lag problem

Hello everyone i'v been having this crazy lag today in raid where i cant do anything use any spell or anything was doing rs25 and the lag only starts when i start the boss fight i dont really know if its a internet problem or not because before i start the boss fight i didnt lag at all for the first try though i only lagged when the boss fight started where i couldnt use any spell at all and for the second try i lagged a bit i fixed my internet my ms was fine before the boss fight i could use every spell i started the boss fight at first i didnt lag for the first few seconds then i lagged like hell same thing couldnt use any spell at all from lag can anyone help me plz i dont really think its an internet problem though