Here are the docs for SymbolServer:
https://www.julia-vscode.org/SymbolServer.jl/dev/
If I recall correctly, the VSCode Julia extension used to index all packages in an environment locally to get relevant names for autocomplete etc, SymbolServer allows this to be done once in a remote machine from which the result of that indexing is then distributed.
As for startup latency, 3 to 4 minutes sounds extreme but then 2GB DDR3 RAM is very little to run a modern resource hungry editor like VSCode so maybe it’s not that surprising.