Looks like the diagnostics system (lint_r_source_document) may be the culprit here.
If you disable diagnostics, does the issue go away? (Try unchecking the [*] Show diagnostics for R checkbox)
If this does fix the issue, it implies that there's something in your R code that's causing the diagnostics system to slow down. Would you be able to share the document(s) you are working with where you're seeing this slowness?