My PI running OctoPrint reports throttling on occasion. This page explains it and how to diagnose it:

Basically, use the

vcgencmd get_throttled

command to show the status. Bits are interpreted as:

Bit  Hex Value  Meaning
 0       1      Under-voltage detected
 1       2      ARM frequency has been caped
 2       4      Currently throttled
 3       8      Soft temperature limit is active
16    1000      Under-voltage has occurred
17    2000      ARM frequency capping has occurred
18    4000      Throttling has occurred
19    8000      Soft temperature limit has occurred