I understand the timer exists because people didn't want the /summon (horse) to make a significant impact/advantage in RvR, however the only scenario in which reducing the timer would really be impactful is visible soloing.
Small man/8 man /BG tends to always run with speed of hastener or greater provided by speed classes (primary or fallbacks).
I think a much smaller timer (10 seconds after out of combat - no timer after performing actions such as casting) would still ensure it is less advantageous than actual speed classes which would get their speed to kick in immediately on out of combat, and most the speeders would either be not affected (primary speeders are much faster) or are already in such small proportion for solo, it wouldn't matter (runemaster, aug healer, enchanter, warden, sorcerer, theurgist - all very rare to see them out and about soloing).
Small man/8 man /BG tends to always run with speed of hastener or greater provided by speed classes (primary or fallbacks).
I think a much smaller timer (10 seconds after out of combat - no timer after performing actions such as casting) would still ensure it is less advantageous than actual speed classes which would get their speed to kick in immediately on out of combat, and most the speeders would either be not affected (primary speeders are much faster) or are already in such small proportion for solo, it wouldn't matter (runemaster, aug healer, enchanter, warden, sorcerer, theurgist - all very rare to see them out and about soloing).