Southern English grass, fast, path are pronounced in Yorkshire and elsewhere as grass, fast and path.

It's worth noticing that it's usually the vowels which have variants, though sometimes it may be the consonants. For instance, a Scotsman will roll his "r"s, whereas a Londoner won't.

So a lecturer with a particularly strong regional accent will cause non-native speakers considerable difficulty.

Whether a student follows a lecture easily or not depends also on the style of English the lecturer uses.

By style, I mean the type of English chosen to express an idea, at one extreme it may be very formal, at the other colloquial or even slang.

Generally speaking, the more formal the style, the easier it is for the student to understand, for example, a lecturer who says, formally, "this is undoubtedly the writer's central point" will be readily understood. On the other hand if he says, "that's really what the writer's on about", many students will have difficulty in understanding.

Other factors, which I haven't the time to discuss in detail, may also be involved. These include the speed which the lecture is delivered, the rather common use of irony, the peculiarly English sense of humor, references which presuppose a knowledge of British culture, etc.

All these factors combine to make it a formidable task for students to follow lectures comfortably. It's clearly helpful to be aware of the problems and to get as much practice as possible in listening to and trying to understanding spoken English.