Instead of computing text properties for all the text in the buffer, you can arrange to compute the text properties for parts of the text when and if something depends on them.
The primitive that extracts text from the buffer along with its
properties is buffer-substring. Before examining the properties,
this function runs the abnormal hook buffer-access-fontify-functions.
This variable holds a list of functions for computing text properties.
Before buffer-substring copies the text and text properties for a
portion of the buffer, it calls all the functions in this list. Each of
the functions receives two arguments that specify the range of the
buffer being accessed. (The buffer itself is always the current
buffer.)
The function buffer-substring-no-properties does not call these
functions, since it ignores text properties anyway.
In order to prevent the hook functions from being called more than
once for the same part of the buffer, you can use the variable
buffer-access-fontified-property.
If this variable’s value is non-nil, it is a symbol which is used
as a text property name. A non-nil value for that text property
means the other text properties for this character have already been
computed.
If all the characters in the range specified for buffer-substring
have a non-nil value for this property, buffer-substring
does not call the buffer-access-fontify-functions functions. It
assumes these characters already have the right text properties, and
just copies the properties they already have.
The normal way to use this feature is that the
buffer-access-fontify-functions functions add this property, as
well as others, to the characters they operate on. That way, they avoid
being called over and over for the same text.