I use Wireshark will all extcaps expect ssh-based (there is no libssh 0.6 for Ubuntu 12.04) and I think my Wireshark starts in 2s (to see interface list) - demon of speed (splash seems to be not required, but it is fine). What time Wireshark spends for extcaps? It is not matter with startup time like my.
Maybe there is need to add measurement debug logs for extcap (and libpcap!), to see how many time it spends in particular tool/lib and show summary. Extcap may take longest time for some users (for example usage of thousands of extcap tools for some reasons). There is also a problem to ensure which scenario takes longest time: 1. all extcaps + all possible interfaces available [works fast for me]
2. all extcap but no interfaces available (reason like: there is ADB daemon on different port, but there is no androiddump interfaces by default - timeout for connecting for non-existence TCP port seems to be not constant for all platform - however androiddump is fast in this case for Windows [but I see that tree times only])