fetch: cache is_usable_proxy results

fetch.py

@@ -9,6 +9,10 @@ from misc import _log
 
 config = None
 
+# Cache for is_usable_proxy() - avoids repeated validation of same proxy strings
+_proxy_valid_cache = {}
+_proxy_valid_cache_max = 10000
+
 
 def tor_proxy_url(torhost):
     """Generate Tor SOCKS5 proxy URL with random credentials for circuit isolation."""
@@ -616,7 +620,24 @@ def is_usable_proxy(proxy):
     - Malformed strings
     - Invalid port (0, >65535)
     - Private/reserved ranges
+
+    Results are cached to avoid repeated validation of the same proxy strings.
     """
+    # Check cache first
+    if proxy in _proxy_valid_cache:
+        return _proxy_valid_cache[proxy]
+
+    result = _validate_proxy(proxy)
+
+    # Cache result (with size limit)
+    if len(_proxy_valid_cache) < _proxy_valid_cache_max:
+        _proxy_valid_cache[proxy] = result
+
+    return result
+
+
+def _validate_proxy(proxy):
+    """Internal validation logic for is_usable_proxy."""
     try:
         if ':' not in proxy:
             return False