Reacting quickly means what's proportionate and reasonable. This is quite standard wording for a law.

The Act (section 10 about illegal content) says that "In determining what is proportionate for the purposes of this section, the following factors, in particular, are relevant—

(a) all the findings of the most recent illegal content risk assessment

(b) the size and capacity of the provider of a service."

"24 hour coverage" is the maximum that can be achieved so it's not going to be proportionate in many, if not most, cases. People have to ask themselves if it is proportionate for a one-man gardening forum to react within 5 minutes at 3am, and the answer is not going to be "yes".

Obviously you can also automatically hide a flagged submission until it is reviewed or have keywords-based checks, etc. I believe these are a common functionalities and they will likely develop more (and yes, a consequence might be to push more people towards big platforms).

People need to have a calm analysis, not hysteria or politically-induced obtuseness whatever one might think of this Act. If they are a small and not in the UK they can probably completely ignore in any case.