Please, tell me if a noun load and a noun loading are interchangeable? Can I say "fast document load" or "slow image loading"? Are these two words synonyms?
The noun version of loading can be the abstract as commented above, but because it tends to refer to a process rather than an event, it is also natural and common to use it in referring to things that take some time. "Document loading" implies it is(was/will be) ongoing. That is not due to your slow/fast although the effects of those adjectives in your examples reinforce the effect of the verbs.
Load can be a noun or active verb. Noun: "Waiting for the document load to finish" Verb: "Waiting for the document to load"
Loading can be noun or passive verb. Noun: "Document loading in progress" Verb "The document is loading"
I know you only asked about the noun usage, but the verb usages have the same difference. Event vs process.