CURLOPT_LOW_SPEED_LIMIT

#include <curl/curl.h>
CURLcode curl_easy_setopt(CURL *handle, CURLOPT_LOW_SPEED_LIMIT,
                          long speedlimit);

Pass a long as parameter. It contains the average transfer speed in bytes per second that the transfer should be below during CURLOPT_LOW_SPEED_TIME(3) seconds for libcurl to consider it to be too slow and abort.

0, disabled

This functionality affects all supported protocols

int main(void)
{
  CURL *curl = curl_easy_init();
  if(curl) {
    CURLcode res;
    curl_easy_setopt(curl, CURLOPT_URL, "https://example.com");
    /* abort if slower than 30 bytes/sec during 60 seconds */
    curl_easy_setopt(curl, CURLOPT_LOW_SPEED_TIME, 60L);
    curl_easy_setopt(curl, CURLOPT_LOW_SPEED_LIMIT, 30L);
    res = curl_easy_perform(curl);
    if(CURLE_OPERATION_TIMEDOUT == res) {
      printf("Timeout.\n");
    }
    /* always cleanup */
    curl_easy_cleanup(curl);
  }
}

Added in curl 7.1

curl_easy_setopt(3) returns a CURLcode indicating success or error.

CURLE_OK (0) means everything was OK, non-zero means an error occurred, see libcurl-errors(3).

CURLOPT_LOW_SPEED_TIME(3), CURLOPT_MAX_RECV_SPEED_LARGE(3), CURLOPT_MAX_SEND_SPEED_LARGE(3), CURLOPT_TIMEOUT(3)