Now, we have 2019 - March. I'm running the latest version of OSX (10.14.3 (18D109)), Firefox 65.0.1 (64-Bit)
I've got 4 dbs configured for pgAdmin, version 4.3
Opening a Server in a test network, 1 switch away, 1GB network takes more than 40 seconds (I stopped counting...) although the db has only the std schemas. To query for the list of schemas should not take as long as this...
There's clearly something very unusual with your setup - that sort of performance is simply not the norm. For example, I just created a 1 node cluster using EDB's Cloud Database Service on the free trial console, with PG11 running on a t3.small instance in the us-east-1a AWS availability zone. Connecting from my laptop here in the UK (i.e 5000+ miles away) was of the order of about a second - far less than I can realistically time. Expanding nodes on the treeview as I browse the schema is near instant as well. "select * from pg_class" in the query tool returned 342 rows in 526ms.
That's running 4.3 (with a couple of minor - and completely unrelated patches) on macOS Mojave.
So the question is; what's different about your setup? Do you have a large number of objects (tables, functions etc)? Is the slowness when you connect, or when you expand tree nodes (if tree nodes are auto-expanding when you connect, try testing by creating a duplicate server definition with a new name and connecting that, as it will not try to expand nodes then).