Currently, qmlls constructs the Dom twice for each document. This is because qqmlintsuggestions.cpp (the qmlls module that takes care of linting and showing errors and warnings) uses QQmlJSLinter::lintFile() to perform the lint:

Currently, QQmlJSLinter::lintFile() does create a new Dom instance to perform its linting, but it should reuse the Dom created by QQmlCodeModel::newDocForOpenFile(). Same for all dependencies required for the Dom construction: QQmlJSResourceFileMapper, QQmlJSImporter, the QQmlDomAstCreator and QQmlJSImportVisitor inside of QQmlDomAstCreatorWithQQmlJSScope, etc.... See also createDom() in qqmldomastcreator.cpp and compare it to QQmlJSLinter::lintFile().