Here is my setup

tsconfig.json with the following content:
{
"compilerOptions": {
"module": "amd",
"noImplicitAny": true,
"removeComments": false,
"preserveConstEnums": true,
"target": "ES5",
"sourceMap": false
}
}

Our workspace has around 900 ts files which are all covered by the above tsconfig.json

Steps to reproduce:

• open any file is such a workspace
• request completions via the tsserver
• type a character in that file
• request completions again.

Observe: fulfilling this request takes between 1800 and 2000 ms on my Lenevo T430s

I debugged it a little and around 90% of the time is spend in sznchronizeHostData, and there in createProgram. It looks like that all files listed in a tsconfig.json are treated as root files and changing one file flushes the whole compiler.