diff --git a/_extensions/quarto-ext/shinylive/shinylive.lua b/_extensions/quarto-ext/shinylive/shinylive.lua
index a8ce817..1e8f970 100644
--- a/_extensions/quarto-ext/shinylive/shinylive.lua
+++ b/_extensions/quarto-ext/shinylive/shinylive.lua
@@ -40,16 +40,153 @@ function throw_quarto_error(err_msg, ...)
   assert(false, err_msg .. "\n")
 end
 
+-- ===== Shinylive subprocess result cache =====================================
+-- Memoizes subprocess results to disk so they are shared across documents
+-- within a full-project render. Without this, every document re-runs several
+-- identical subprocesses (`extension info`, `extension base-htmldeps`,
+-- `extension language-resources`, plus two per code block), and their
+-- interpreter startup cost dominates large-site render times.
+--
+-- Results are only cached once the live version of the tool that produced
+-- them is known (see the `keySuffix` parameter of `cachedPipe`): every cache
+-- key embeds that version, so upgrading the shinylive package (or quarto)
+-- naturally invalidates the cache and stale reuse is impossible.
+--
+-- The cache is strictly an optimization: on any cache failure, the real
+-- subprocess is run.
+
+local pipeCacheState = nil -- nil = uninitialized, false = disabled, table = ready
+
+local function initPipeCache()
+  if pipeCacheState ~= nil then
+    return pipeCacheState
+  end
+  local ok, result = pcall(function()
+    local projDir = quarto.project.directory or os.getenv("QUARTO_PROJECT_DIR")
+    if projDir == nil then
+      return false
+    end
+    local dir = tostring(projDir) .. "/.quarto/shinylive-cache"
+    pandoc.system.make_directory(dir, true)
+    return { dir = dir }
+  end)
+  if ok and result then
+    pipeCacheState = result
+  else
+    pipeCacheState = false
+  end
+  return pipeCacheState
+end
+
+local function pipeCacheKey(command, args, input, keySuffix)
+  return pandoc.utils.sha1(
+    command .. "\1" .. table.concat(args, "\1") .. "\1" .. input .. "\1" .. keySuffix
+  )
+end
+
+local function pipeCacheRead(cache, key)
+  local f = io.open(cache.dir .. "/" .. key, "rb")
+  if f == nil then
+    return nil
+  end
+  local contents = f:read("*a")
+  f:close()
+  return contents
+end
+
+-- Some subprocess results embed absolute paths to files elsewhere on disk,
+-- and running the subprocess is what creates those files: notably
+-- `extension base-htmldeps` downloads the shinylive web assets to the user
+-- cache dir as a side effect. Only serve a hit whose referenced files all
+-- still exist; otherwise re-run the subprocess so it can re-create them.
+-- (Matches POSIX absolute paths in "source"/"path" JSON fields; on Windows
+-- nothing matches, so hits are served unvalidated — same as no check.)
+local function pipeCacheHitUsable(hit)
+  for _, field in ipairs({ "source", "path" }) do
+    for path in hit:gmatch('"' .. field .. '":%s*"(/[^"]+)"') do
+      local f = io.open(path, "r")
+      if f == nil then
+        return false
+      end
+      f:close()
+    end
+  end
+  return true
+end
+
+local function pipeCacheWrite(cache, key, value)
+  -- Write to a temp file then rename: atomic on POSIX, so a concurrent
+  -- render (e.g. quarto preview) can never observe a partial entry.
+  local path = cache.dir .. "/" .. key
+  local tmpPath = path .. ".tmp" .. tostring(math.random(1000000000))
+  local f = io.open(tmpPath, "wb")
+  if f == nil then
+    return
+  end
+  f:write(value)
+  f:close()
+  os.rename(tmpPath, path)
+end
+
+local pipeMemo = {} -- in-process memo: repeat calls skip hashing and disk I/O
+
+-- Drop-in replacement for `pandoc.pipe()` that memoizes successful results
+-- to disk. `keySuffix` must encode the live version of the tool being run
+-- (and anything else the result depends on besides command/args/input); pass
+-- nil to skip caching entirely — used for the version-discovery calls
+-- themselves, which therefore still run once per document and keep the cache
+-- honest. Failed subprocess calls are never cached (pandoc.pipe raises, so
+-- the error propagates to the caller exactly as before).
+local function cachedPipe(command, args, input, keySuffix)
+  if keySuffix == nil then
+    return pandoc.pipe(command, args, input)
+  end
+  local cache = initPipeCache()
+  if cache == false then
+    return pandoc.pipe(command, args, input)
+  end
+
+  local keyOk, key = pcall(pipeCacheKey, command, args, input, keySuffix)
+  if not keyOk then
+    return pandoc.pipe(command, args, input)
+  end
+
+  if pipeMemo[key] ~= nil then
+    return pipeMemo[key]
+  end
+
+  local hitOk, hit = pcall(pipeCacheRead, cache, key)
+  if hitOk and hit ~= nil then
+    local usableOk, usable = pcall(pipeCacheHitUsable, hit)
+    if usableOk and usable then
+      pipeMemo[key] = hit
+      return hit
+    end
+  end
+
+  local res = pandoc.pipe(command, args, input)
+  pipeMemo[key] = res
+  pcall(pipeCacheWrite, cache, key, res)
+  return res
+end
+-- ===== End shinylive subprocess result cache =================================
+
 -- Python specific method to call py-shinylive
 -- @param args: list of string arguments to pass to py-shinylive
 -- @param input: string to pipe into to py-shinylive
 function callPythonShinylive(args, input)
   -- Try calling `pandoc.pipe('shinylive', ...)` and if it fails, print a message
   -- about installing shinylive python package.
+  -- Cache results once the live package version is known; the `--version`
+  -- discovery call itself runs before that and is never cached.
+  local keySuffix = nil
+  if pyShinyliveVersion ~= nil then
+    keySuffix = "py-shinylive-" .. pyShinyliveVersion
+  end
   local res
   local status, err = pcall(
     function()
-      res = pandoc.pipe("shinylive", args, input)
+      res = cachedPipe("shinylive", args, input, keySuffix)
     end
   )
 
@@ -75,10 +212,16 @@ function callRShinylive(args, input)
 
   -- Try calling `pandoc.pipe('Rscript', ...)` and if it fails, print a message
   -- about installing shinylive R package.
+  -- Cache results once the live package version is known; the initial
+  -- `extension info` call that discovers it is never cached.
+  local keySuffix = nil
+  if versions.r ~= nil then
+    keySuffix = "r-shinylive-" .. versions.r.version
+  end
   local res
   local status, err = pcall(
     function()
-      res = pandoc.pipe("Rscript", args, input)
+      res = cachedPipe("Rscript", args, input, keySuffix)
     end
   )
 
@@ -424,10 +567,19 @@ return {
       ensureLanguageSetup(language)
 
       -- Convert code block to JSON string in the same format as app.json.
-      local parsedCodeblockJson = pandoc.pipe(
+      -- The result depends on the codeblock-to-json script (which ships with
+      -- the language's shinylive package) and the quarto CLI that runs it, so
+      -- key the cache on both of their versions.
+      local codeblockKeySuffix = nil
+      if versions[language] ~= nil then
+        codeblockKeySuffix = language .. "-shinylive-" .. versions[language].version ..
+            "\1quarto-" .. tostring(quarto.version)
+      end
+      local parsedCodeblockJson = cachedPipe(
         quarto_cli_path,
         { "run", codeblockScript, language },
-        el.text
+        el.text,
+        codeblockKeySuffix
       )
 
       -- This contains "files" and "quartoArgs" keys.