TCPIP vs external interrupt

Why running TCPIP inserts random delays into the external interrupt response time. External interrupt has priority 7 and uses SRS. Nothing else with priority 7. The TCPIP server is a Harmony example.
If the server is not running, the external interrupt delay is constant (1us).
After starting the server, it changes between 1 - 2 us.
I do not know what to try anymore.

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