Regularly get "This merge is too large to display." message in pull requests, that aren't really that big.

We are increasingly seeing this message when trying to view the diffs in a pull request. In some cases refreshing the browser a couple of times seems to get it to load properly. But in some cases it just repeatedly comes up with the above message.
The pull requests do not contain an excessive number of changes (may be 40 files) and none of the files have that many diffs.
It seems as though if the request takes more that about 15 seconds it just gives up and says it is too large; this makes the tool unusable if it imposing such a low limit on how large our changes can be.
Can there not be some configuration on the time out? Or better still it return the list of files and allow each to be clicked on separately to display the diffs one file at a time, in cases where it takes too long to process the whole thing.