Dock Pulses: It's Actually TWO apps opening and being closed continuously

I believe "WAAppKitBridgeService" is a relic from "WhatsApp", of which two versions appear to be installed.

The general "lag and delay in responsiveness" observation is probably unrelated, and likely due to the presence of various and sundry "anti-virus" junk installed on that Mac, of which several products can be contributing factors. Uninstall anything and everything resembling that description. Rule 1 of Macs is don't install junk.

VPNs don't help either: Don't use VPN services.

In each case follow their respective uninstallation instructions, observe the Mac's performance after relieving it of all those unnecessary hindrances, and post a new EtreCheck report.

Two apps attempted to load, but their icons bore a "prohibited symbol" which simply means they will not run on that macOS version. It is not possible to determine what they may be from that video.

To advance troubleshooting please download and run EtreCheck, then post its report in a reply to this Discussion.

Instructions: How to use the Add Text Feature When Posting Large Amounts of Text, i.e. an Etrecheck Report - Apple Community. Please take the time to read and follow those instructions with care.

Do as suggested by John Galt in his post above.