CPU frequency gets throttled when using multi-window on Kubuntu 18.10

This is the current CPU frequency with power plugged in:

$ cpufreq-info | grep -i "current CPU freq"
  current CPU frequency is 1.54 GHz.
  current CPU frequency is 1.58 GHz.
  current CPU frequency is 1.73 GHz.
  current CPU frequency is 1.83 GHz.

I tried uninstalling TLP, and it doesn't work. Unplugging sometimes brings back the performance, but other times, it doesn't and requires a reboot (like right now).

After reboot (with only 1 monitor):

$ cpufreq-info | grep -i "current CPU freq"
  current CPU frequency is 2.95 GHz.
  current CPU frequency is 2.79 GHz.
  current CPU frequency is 2.85 GHz.
  current CPU frequency is 2.78 GHz.

I'm just wondering if this is happening to anyone else?

This is my system info (copied from KInfoCenter):

Operating System: Kubuntu 18.10
KDE Plasma Version: 5.15.2
KDE Frameworks Version: 5.54.0
Qt Version: 5.11.1
Kernel Version: 4.18.0-15-generic
OS Type: 64-bit
Processors: 4 × Intel® Core™ i7-4510U CPU @ 2.00GHz
Memory: 15.6 GiB of RAM

Any suggestions on how to fix it? The only one I tried out is uninstalling TLP, which (as mentioned before) didn't work.