Is possible. However, since it is set to the fastest speed in the initial state, it is possible to reduce the speed.

The configuration steps in the Silicon Labs IDE are as follows.

1. Select Options ⇒ Connection Options.
2. Click C2 Clock Strobe Configuration at the bottom of the Connection Options window. "C2 Clock Strobe Configuration" may not be displayed if the Silicon Labs IDE is not the latest. In that case, refer to the Silicon Labs community site and call it with CTRL+ALT+S.
3. The C2 Clock Strobe Time window allows you to change the C2CK Low time.

Note that C2CK does not have a 50% duty cycle (high/low times are the same), so be careful when calculating the frequency.

When the C2 Clock Strobe Time is set to the minimum of 0.25us (default), the measured frequency is about 950kHz (top of the figure below), and when it is set to the maximum of 5us, the frequency is about 170kHz (bottom of the figure below). (Using Silicon Labs IDE ver.5.10)