Man pages matching name curlinfo_speed_download_t in section 3:

CURLINFO_SPEED_DOWNLOAD_T.3.en	debian 10 / libcurl4-doc
CURLINFO_SPEED_DOWNLOAD_T.3.en	debian 11 / libcurl4-doc
CURLINFO_SPEED_DOWNLOAD_T.3.en	debian 12 / libcurl4-doc