FIXED!!!! Opened task manager and noticed my CPU speed was at 0.39Ghz which didn't look right. Googling that figure threw up lots more results. Downloading 'ThrottleStop' fixed the problem within seconds.
A really annoying problem that has only started in the last few days...
If I have my laptop plugged into the mains, it's great. As soon as I unplug it, every slows down. Things take ages to open and my internet speed takes a massive hit (a speed test with the power plugged in gives me 200Mbps, as soon as I unplug it I get about 50Mbps!)
I have Googled and played with all the settings i.e. checking power management and making sure it's all set up for maximum performance.
Another issue I have noticed is that my battery indicator now stays at '100% charged' even though it isn't. I have done various battery diagnostics using the Windows tools, but they don't flag any issues up.
I've literally just 'reset' the laptop but chose the option to retain my files. This has made no difference even though it seems to have rolled back on some of the windows updates which I thought might have caused the problem in the first place.
Any ideas? Tearing my hair out! With the cable plugged in, the laptop is great, without the cable it's like I'm using a 30 year old machine!