Is it feasible to use powerbank for 100W speaker
You'll get best results by using different strategies to maximize "loud time" depending on the content:


If this is a single musical instrument (e.g. a guitar) then your best bet would be to find a high-efficiency speaker (105 dB/W @1m is not uncommon for guitar speakers) and a high-efficiency guitar amp like this one.  Most of the efficiency of this setup comes from not amplifying the frequencies outside of the guitar's range.  This same general approach works for different single instruments (and voice) with some tweaking of the amplifier and speaker.
On the other hand, if you're doing a sort of PA system where the sound is full-spectrum (e.g. prerecorded music), you'll want to reproduce a broader range of frequencies, and you'll pay for that in terms of either overall loudness or battery life.
Low (bass) frequencies require a lot more energy in order to be perceived as loud, so some careful selection of program material can help extend your run-time.  Choose content with short bursts of bass rather than long drawn-out bass.
Finally, adjust your pre-amplifier equalizer settings to avoid amplifying (especially bass) frequencies that either (a) don't match your speaker or (b) don't sound good in the space.  A lot of "good sounding" small speakers (Bose is famous for this) make heavy use of this technique.


One last thought: depending on the loudness you're aiming for, some of the newer wireless speaker systems might be sufficient, even though their rated wattage might be more like 5-20.  The correlation between wattage and perceived loudness is roughly logarithmic, and so a (huge, power hungry) 100W amp is only about 4x louder than a 1W amp.