Routerbord 1100 v5.18 with 100% of CPU

I have a Routerbord 1100 in V5.18 and 1,5 Mb of memory. In the last week the number of user online increased to 300 and the use of CPU is going from 46% up to 100%. I would like to know what coulb be causing this situation.

you can use ‘/tool profile’ to see what is causing issues. However my suggestion would be to upgrade router to 5.22 as there where some issues in 5.18 or 5.19 that caused high CPU load without any reason (problem was resolved in later releases)