As you approach the junction, access to the M25 is via the inside lane, which at this point is still full of HGVs heading for the Channel. So it's a slow slog. Or - you could wait to the very end, just when some foreign HGV driver realises he needs to pull out into the middle lane to continue on the M26 - oops too late! Or, as is the case with those unfamiliar with the junction, "surely I don't have to turn off the M25 to stay on the M25?" - "damn... I do!" - and veers straight over the hatched area onto the M25 slip - just. Yep, I've seen all of the above, and they happen on a scarily regular basis.
Finally to cap it all, the slip goes uphill, it divides into two lanes just before the exit. So the inner lane tends to be slow, while the outer is much faster. The inner continues on and becomes the third and innermost lane of what was the two lane A21, whilst the now faster moving traffic feeds into the slower inner lane of the former A21! Great!