I have just upgraded from PhpED 19.116 to 19.2 and I find that 19.2 is much slower to initially load when started; 19.1x was much faster by comparison. Has anyone else noticed this? It loads so slowly by comparison it feel like the apps that use all the .NET assemblies, rather than compiled Visual C redistributables.

It does not use any assemblies at all and it can't be any slower than 19,1 or 19.0 versions.
But _in_the_beginning_ it re-builds code insight for declaration stubs shipped with product and all your projects sources you're loading. It requires time and resources, of course, but corresponding slowdown is normally within 10-20% and wouldn't last for more than an hour for very very large projects.
What can bring more dramatic results is antivirus running in active mode. It may dramatically slow down the product tracking all its activity. In this case you can try to whitelist PhpED in your AV or in other words tell it you trust us and check if it helps.

I've just upgraded to version 19212 -- this most recent version loads as quickly as the older (pre 19.2x) versions -- problem solved!

I'm sorry, but 19212 does _not_ bring any changes, other than a hot fix / workaround that addresses issue with debugger.
So if it works differently, it must be something with your system, (e.g antivirus), or, which is also possible -- previously running binary was the one you installed as an upgrade from a previous version, so it had to re-index and re-parse all the stubs it brought and your projects too. This process could take resources, then after it finished and started working faster, you installed 19212 and attributed all the advantages to this build.

I agree that other factors (that you've outlined) may have been the culprit(s) re slow loading -- none the less I'm pleased that PhpED works as it has it the past for me.