Or HaChaim on Exodus 5:8/אור החיים על שמות ה׳:ח׳

1 כי נרפים הם, "for they are lazy!" Who had ever suggested that the idea to sacrifice to their G'd originated in the relatively light burden of work that Pharaoh had imposed on the Israelites up until then? Perhaps what Pharaoh meant was that the Israelites felt that now that Moses had arrived their lot would become easier and that therefore they wanted to thank their G'd for that by offering up thanksgiving sacrifices so that He would cancel out the servitude altogether. Pharaoh countered this euphoria by making the workload heavier so that they would not even entertain any idea of their servitude being near an end, i.e. those lies that Moses had fed them.

2Pharaoh also reacted to the Israelites having said: נלכה נזבחה, "let us go slaughter, etc." He understood this as an implied promise that the Israelites planned to make up the time lost by working extra hard after their return. He reasoned that if they could really do this it was proof they had not been working hard enough thus far. This is why he withdrew the supply of straw which had been provided up until then and referred to them as lazy. Eventually, their holiday would backfire and the would cry out, i.e. צועקים that they had to work too hard. Pharaoh wanted to head off all these complications by ordering now that the straw would be withdrawn so that the people would stop dreaming about all kinds of improvements in their lives.

3The expression ואל ישעו reflects weakness. Pharaoh interpreted the people's weakness as stemming from the "illusions" (in his view) which Moses had come to feed them. As a result they had expressed fear of punishment by their G'd if they did not offer the sacrifices their G'd demanded. When people live in fear they are weak and it undermines their ability to perform good work. Pharaoh wanted to stop this process at the source, hence his decree to suspend the deliveries of straw.