If the Cisco router lags, how can we troubleshoot it without restarting?

Check CPU utilization and follow the Cisco guidelines on that.

hi how r u , you can tell us the series of router ? , and what king of hang (lag,stop) , and console cable response or not ? , some of network  management working or not (telnet,,,,) thanks

There are few things which we need to check whenever router is lags/slow in response

Check the CPU utilization by using "sh proc cpu" command check which process consuming more

Check the memory utilization and look for free space (if uptime is very high then probably need to plan a reboot to free up its memory cache)

Check for any loops and any process by keeping on chocking router resources (example multicast packet with TTL <1)

lags will rarely happens >> and if its happen we just go for restart

If the console does not respond because the router CPU utilization is high, it is important to find and correct the cause of the high CPU utilization. For example, if process-switched IP traffic causes problems, then this is reflected in the "IP Input" process in the output from the show processes cpu command. In this situation, it is important to collect the output from show interfaces, show interfaces stat, and possibly show processes to further diagnose the problem. To fix the problem, you would need to reduce the amount of IP traffic that is process switched

Try performing a Ping test to see if you can connect to the outside world. There are several free Ping utilities available, but the easiest way to ping a website using Windows is to open a command prompt and type "Ping" followed by the IP address of the site you want to ping. If you don't know the IP address, you can type the full address. For example, to ping Google, type Ping google.com. This sends small data packets to the target site out and measures how fast your connection is in milliseconds. If the test is successful, you'll see timed results. If it continuously fails, try pinging several sites. Keep in mind that not all websites accept pings. If you still can't get results, you might have a problem with your modem or router, or the problem might be on your service provider's end.

check running config.

check log

show errors on the interfaces

cisco router never lags unless it ws disconnected