I'm sure things could be improved, as with anything, but if there are still 100 outside at 3:10pm it is probably safe to say at least another 100 got through in the fifteen minutes preceeding this? That means at least 15% of the crowd trying to get in five minutes before kick off.
This volume/percentage of total crowd would cause timely entry issues anywhere. Replicate this up the road and do you think it'd be any smoother if 12k fans turned up five minutes before kick off at Old Trafford? They wouldn't all get in for kick off either, the same for any sports arena/show/cinema etc.
It's personal choice what time anyone gets to the ground or drinks beforehand, but it can't be expected that volunteers man additional turnstiles only from 2:50pm onwards, when that appears to be the only time there is a bottleneck and problems with speedy entry. For the size of attendance, the number of open turnstiles seems to be fine until the last minute rush.