Read the passage carefully and answer the question. Throughout the night we continued to feel our way southward along the left bank of the river, and by four o’clock reached a village which was…

Read the passage carefully and answer the question.

Throughout the night we continued to feel our way southward along the left bank of the river, and by four o’clock reached a village which was temporarily deserted; this was fortunate for us as thanks to our incompetent guide, we plundered into it without any warning. It was only five miles north of Myo, where a tributary stream entered the river from the east. We still had four miles to cover before meeting the boatman who had promised to take us across to the comparative safety of the right bank.
In the two hours before dawn, therefore, there was much to be accomplished and many difficulties had to be surmounted. We had long since decided that our guide was more of a liability than an asset; our rate of progress along an unknown track was unavoidably slow; the good faith of the boatman was problematical; the river, normally two hundred yards wide at the point of crossing, had been swollen by the recent rains. On the other hand, the men were in good heart and excellent physical condition; the rain had stopped and the moon no longer obscured by the black clouds, offered her friendly light when we most needed it; the enemy did not patrol the left bank above the confluence at Myo. The odds, so overwhelmingly against us when we started our march, were now in our favour.

What is stated to be ‘unavoidably slow’?