I know my laptop is a little slow to start up because I have quite a few things going on all at once, especially Avira trying to update before my wifi is even fully connected to my router. That generally pegs both cores of my CPU at 95–100% for about two minutes as it tries to sort through everything. However, it doesn’t slow down after that unless I keep it on long enough to have stuff try to update again or unless one of my security programs decides to do a sweep in the background.
One thing that happens to a lot of people is that they run out of memory and have to start flogging the Virtual Memory which is about 1,000 times slower. My laptop has 3GB of RAM and my desktop has 6GB so I personally have not had that problem on these computers, but a lot of people with older computers that have 2GB or less run into it quite a bit and anybody with 1GB or less running any software made in the last 10–15 years (like any version of Windows after Win95) runs into it as soon as they boot up.
Bad memory modules are uncommon, but I have had a couple of situations where a bad DIMM caused issues as soon as the memory filled up to a certain point. I have also had a few throttle down the CPU due to overheating when the heat sink was clogged with cat hair or I just didn’t give my computer enough ventilation.