Other countries aren’t the issue (and in fact switzerland mostly stopped allowing german trains into their network because they were the main reason for delays).
Japan has dedicated tracks for their shinkansen (and afaik for low speed passenger trains as well), whereas in germany ICE, IC, all regional trains, and freight trains share the same network. Different speeds on the same track lead to delays much faster as trains have to wait to be overtaken etc. The network is also operating above capacity (which has the obvious upside of better coverage/more frequent trains, but still we need more capacity). Many parts of the network are also simply old and have not been maintained well - we still have some manually operated signals ffs.
None of this is easy to fix and overall the railway network is still very good in an international comparison, but the neglect of the past 30 years is very obvious and has led to a lot of problems.
Other countries aren’t the issue (and in fact switzerland mostly stopped allowing german trains into their network because they were the main reason for delays).
Japan has dedicated tracks for their shinkansen (and afaik for low speed passenger trains as well), whereas in germany ICE, IC, all regional trains, and freight trains share the same network. Different speeds on the same track lead to delays much faster as trains have to wait to be overtaken etc. The network is also operating above capacity (which has the obvious upside of better coverage/more frequent trains, but still we need more capacity). Many parts of the network are also simply old and have not been maintained well - we still have some manually operated signals ffs.
None of this is easy to fix and overall the railway network is still very good in an international comparison, but the neglect of the past 30 years is very obvious and has led to a lot of problems.