Sorry, bit slow today, who said PC ? Just as long as no one ever mistakenly associates that as politically correct for me.

Yes they often slow down. It can be for dozens of reasons so the issue is which one.

You need to have >8Gb RAM to be effective on Win10.
If the laptop/PC is old the hard disc (C:) is a mechanical spinning thing and quite slow.
It gets slower as it gets full as well so if you have less than 10-15% of disc space left assume this does not help.
Replacing this with an SSD of 512Gb will be about £50 and be a big improvement. You need to move the operating system and licences so it needs swapping correctly.

Other things.
Remove old software you don't use.
Make certain that you only have one anti-virus program and not old ones loaded as well.
The built in windows defender and firewall software is actually pretty good now so you don't need to spend £40 on aftermarket stuff unless you do visit questionable sites.....

More than happy to help if there is anything specific?

Everyone loves a Morgan. Even me, unless it's broken again.