Those warnings about resource shortages have been added about a year ago in Dec 2021 with Pi-hole FTL v5.12, Web v5.9 and Core v5.7 released.
Searching this forum for issues on system load returns a number of hits, e.g. Getting Load issue in pihole running in VM when different instance of pihole in pizero does not have this issue - #4 by jfb or Long-term load (15min avg) larger than number of processors: 4.4 > 4 - #2 by DL6ER.
Likewise, searching our docs yields the respective CHECK_LOAD option.