Agree with @purism121, this sounds like it could be related to this issue. When this happens for me I’ve noticed that the CPU frequency is pegged at 400Mhz. Next time you wake up the system and it seems slow open up a terminal window and run
watch grep MHz /proc/cpuinfo
and note what the values are around. If they’re around 400MHz it’s not sluggish because PureOS is working too hard, but because the CPU isn’t allowed to run at its full speed.