Indeed, it is the very same issue with the route over a Deutsche Telekom node in the USA. Your origin server is located in Poland?
Not sure about the chances, but it could make sense to contact your ISP about this issue. I mean Cloudflare is very widely used, and it seems that every Deutsche Telekom (or daughter) customer in Europe who accesses any origin server located in Europe behind Cloudflare free plan edge, has this issue. That might be quite many cases. Of course visitors usually would expect the website provider to have an issue, not the ISP, when experiencing slow page loads, so DTAG might not have much motivation to invest time. But the chances are probably better compared to asking Cloudflare for a solution.
Btw, adding some info: We sadly forgot to check the edge servers (IPs) we were offered on the free plan, but after switching to pro plan, all of us get the same 3 IPv4 and 3 IPv6 addresses offered from DNS, which differ from the two IPs from the two traceroutes I posted before. But the subnets overlap. So it seems like our proxied domains are now getting a different set of Cloudflare edge servers from the same subnets. What I was wondering about is that this change happened within a minute. I literally hit the pro plan button, got a confirmation text immediately, told my team in a video chat that I did that, and they had proper low response times and the different route immediately. This was faster than the 5 minutes (auto) TTL that those DNS entries should have 
: Time to Live (TTL) · Cloudflare DNS docs
Proxied records
By default, all proxied records have a TTL of Auto, which is set to 300 seconds.