Having this exact same issue on a Dell Latitude 7400 and I can now re-create and terminate this issue at will. Once the issue starts, the system freezes for about one second every 30 seconds.
To cause the issue to start occurring: While the laptop lid is open and the laptop is connected to an external monitor over HDMI, while logged into the Cinnamon desktop, close the laptop lid.
To cause the issue to stop occurring: Open the laptop lid again, or log out and back into the desktop WITHOUT opening the laptop lid, otherwise the problem will start again.
So the cause is closing the laptop lid. As the OP said, it doesn't matter if the laptop's main display is disabled or not. Either opening or closing the laptop lid will make the computer freeze for one second, but if left open the issue will go away, and if left closed the issue will stick around unless you log out and back in or open the lid.
It seems like whatever is being triggered upon closing the laptop's lid perhaps thinks it failed to do whatever it was trying to do, and re-tries to do that thing every 30 seconds? I assume this is a graphics driver issue, Xorg issue, or power/hibernation/suspending issue of some sort. I have "When the lid is closed: Do nothing" set. Systemd's hibernation, suspend, and suspend-then-hibernate targets are all unmasked. Since the laptop is a different model from the OP's, this may be a more widespread issue (same i915 driver). My specs:
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System: Kernel: 5.13.0-22-generic x86_64 bits: 64 compiler: N/A Desktop: Cinnamon 5.0.7 wm: muffin 5.0.2 dm: LightDM 1.30.0
Distro: Linux Mint 20.2 Uma base: Ubuntu 20.04 focal
Machine: Type: Laptop System: Dell product: Latitude 7400 v: N/A serial: <filter> Chassis: type: 10 serial: <filter>
Mobo: Dell model: 07WDVW v: A00 serial: <filter> UEFI: Dell v: 1.3.11 date: 06/11/2019
CPU: Topology: Quad Core model: Intel Core i7-8665U bits: 64 type: MT MCP arch: Kaby Lake rev: C L1 cache: 256 KiB
L2 cache: 8192 KiB L3 cache: 8192 KiB
flags: avx avx2 lm nx pae sse sse2 sse3 sse4_1 sse4_2 ssse3 vmx bogomips: 33599
Speed: 4800 MHz min/max: 400/4800 MHz Core speeds (MHz): 1: 4800 2: 4785 3: 4323 4: 4238 5: 4428 6: 4498 7: 4303
8: 4796
Graphics: Device-1: Intel UHD Graphics 620 vendor: Dell driver: i915 v: kernel bus ID: 00:02.0 chip ID: 8086:3ea0
Display: server: X.Org 1.20.11 driver: modesetting unloaded: fbdev,vesa resolution: 1920x1080~60Hz
OpenGL: renderer: Mesa Intel UHD Graphics 620 (WHL GT2) v: 4.6 Mesa 21.0.3 direct render: Yes
Audio: Device-1: Intel Cannon Point-LP High Definition Audio vendor: Dell driver: N/A bus ID: 00:1f.3 chip ID: 8086:9dc8
Device-2: JMTek LLC. USB PnP Audio Device type: USB driver: hid-generic,snd-usb-audio,usbhid bus ID: 1-2:3
chip ID: 0c76:161f
Sound Server: ALSA v: k5.13.0-22-generic
Network: Device-1: Intel Wi-Fi 6 AX200 driver: iwlwifi v: kernel port: efa0 bus ID: 3a:00.0 chip ID: 8086:2723
IF: wlp58s0 state: up mac: <filter>
IF-ID-1: docker0 state: down mac: <filter>
IF-ID-2: enx9cebe8d1ad04 state: down mac: <filter>
Drives: Local Storage: total: 253.38 GiB used: 42.68 GiB (16.8%)
ID-1: /dev/nvme0n1 model: SSDPEMKF256G8 NVMe INTEL 256GB size: 238.47 GiB speed: 31.6 Gb/s lanes: 4
serial: <filter> rev: 7004 scheme: GPT
ID-2: /dev/sda type: USB vendor: SanDisk model: Cruzer size: 14.91 GiB serial: <filter> rev: 1.19 scheme: MBR
Partition: ID-1: / size: 88.34 GiB used: 41.96 GiB (47.5%) fs: ext4 dev: /dev/dm-1
ID-2: /boot size: 739.9 MiB used: 123.4 MiB (16.7%) fs: ext4 dev: /dev/nvme0n1p5
Sensors: System Temperatures: cpu: 55.0 C mobo: N/A
Fan Speeds (RPM): cpu: 0
Repos: No active apt repos in: /etc/apt/sources.list
Active apt repos in: /etc/apt/sources.list.d/alexlarsson-flatpak-focal.list
1: deb http://ppa.launchpad.net/alexlarsson/flatpak/ubuntu focal main
Active apt repos in: /etc/apt/sources.list.d/docker.list
1: deb [arch=amd64 signed-by=/usr/share/keyrings/docker-archive-keyring.gpg] https://download.docker.com/linux/ubuntu focal stable
Active apt repos in: /etc/apt/sources.list.d/lutris-team-lutris-focal.list
1: deb http://ppa.launchpad.net/lutris-team/lutris/ubuntu focal main
Active apt repos in: /etc/apt/sources.list.d/official-package-repositories.list
1: deb http://packages.linuxmint.com uma main upstream import backport #id:linuxmint_main
2: deb http://archive.ubuntu.com/ubuntu focal main restricted universe multiverse
3: deb http://archive.ubuntu.com/ubuntu focal-updates main restricted universe multiverse
4: deb http://archive.ubuntu.com/ubuntu focal-backports main restricted universe multiverse
5: deb http://security.ubuntu.com/ubuntu/ focal-security main restricted universe multiverse
6: deb http://archive.canonical.com/ubuntu/ focal partner
Info: Processes: 340 Uptime: 31m Memory: 15.44 GiB used: 2.06 GiB (13.4%) Init: systemd v: 245 runlevel: 5 Compilers:
gcc: 9.3.0 alt: 9 Shell: bash (sudo) v: 5.0.17 running in: terminator inxi: 3.0.38
