High Sierra system size

My Mac-book has High sierra with 1 TB Hard Disk Size , 8 GB Ram but the performance now is very slow and it;s told me that i have no space for doing any action on it , after that i check the storage space and i found that only the system take 700 GB which is too much for any system , any way when i'm trying to delete some files from my own data, i deleted about 200 GB and checked that it's deleted also from Trash but in the other hand i found that nothing gonna happen with spacing size and still faced same small spacing size with same issue so please advise what is that ???