this question does not have a single simple answer.
theoretically, 30mbps = 3.75MBps = 225MB/min =13.5GB/hr
that speed can easily get u a full HD video, theoretically.
in fact, that speed will give u uhd4K dolby digital 5.1 with netflix , no problem.
but... whether kodi or other apps, the factors incl:
.. yr incoming may not be stable or consistent.
.. yr wifi connectivity may be poor.
.. the traffic may be heavy.
.. there may be geoblocks.
.. there may be local ISP blocks.
.. the final site that the link goes to is poorly maintained.
.. yr device hardware/chipset/firmware/memory may be poor
i use an androidtv 5.1 on wifi and an android box 4.4 on ethernet, both with ample memory - similar performance.
generally speaking, i do not have problems with SD whether kodi or other apps.
for HD, standalone apps, a lot of them work well in HD.
for HD, there are far fewer HD addons/links/sites that work reliably on KODI.
to me, that is becos KODI is more of a free-for-all platform for enthusiasts to experiment, learn and build, come and go quick.
the standalone apps, tend to be more professional and hold out much better and longer.
Thanks for the advice, i have no issue when streaming movies/drama on 1080 quality (exodus, ICdrama, Carton HD...).
buffering only happen when i stream LIVE TV (Oblivion) such as CNBC, CNN, HBO HD and so on.....
how is your LIVE TV experience? care to share which app or addon you watch LIVE TV?