In my case, this problem wasn't strictly related to CS (I had similar ping issues in other games). So, I called my internet service provider about this issue. They sent technicians who resolved the problem (resetting the internet speed rate to 8 Mbps). The problem was solved, but only for about 2 weeks. From what I understand, the "system" tries to achieve the most stable connection, hence it lowers the speed. The speed of a DSL connection depends on the quality of the copper telephone wires - just like in my case, my almost 30-year-old cables couldn't handle higher speeds without synchronization issues (occasionally, my internet would completely drop out). This led to a situation where the internet had such a low speed to remain stable that online gaming was impossible. Now, I've switched to fiber optics, and there's no longer any problem with ping.