How can I increase my website speed? (I am already using (w3 total cache)

Hey everyone,

for more than a couple of month I have an issue with my website. My website speed is very very slow. I am working together with some programmers from india unfortunately they can not solve it. They are saying if they change sth. they will destroy the css part and so on. The whole website will be a mess. I am very frustrated and almost giving up on this. Therefore I need your support.
What should I do? I do not want to invest in this too much as well. So I would be very glad about a solution that is cheap but effective.
Website: Paketbeilagenclub
Thank you very much for your support.
with best regards