My Bose Soundsport and Jabra 75t support simultaneous connections. Faster than "almost instantaneous"...because it's connected to 2 devices. I can watch a video on youtube on my laptop and immediately pickup a phone call. I never understood why Apple didn't support this. My Soundsport also annoucnes on startup which devices it's connected to..which is helpful when you have more than 2. My Soundsport is several years old, and Jabra had it since 65t, at least.
Edit: Also apparently this device switching feature for Airpod Pro is good only for Apple devices, and Apple devices on the same iCloud account.
Two simultaneous connections unnecessarily drains the battery of whichever device is not currently being used. It would also provide an inconsistent user experience when using more than two devices (say an iPhone, iPad, and a MacBook). I can see why Apple prefers to make actual switching as seamless as possible rather than support simultaneous connections.
Do you have to choose which device you are listening on by interacting with the headset, or is there a way to toggle that from the source devices?
Edit: Also apparently this device switching feature for Airpod Pro is good only for Apple devices, and Apple devices on the same iCloud account.