I can't see putting the o/s on the flash drive as a good idea, even as a stubby drive, it would be vulnerable to knocks.
If the laptop is worth saving a couple off options are saving off the files you want to keep, and re-installing the o/s. It has been a tried and tested solution for decades. Alternatives, is installing an ssd drive, this will really give a speed boost to disc operations like starting up.
If you've never done it, try some anti malware and anti adware software. Malwarebytes is good for malware
https://www.malwarebytes.com/mwb-download/
and Antivirus
https://www.avg.com/en-gb/free-antivirus-download
There is more you can do, but if you once had a reasonable experience on your laptop in the no too distant past, it may be possible again.
Good luck.
Perhaps if you tell people the make and model you are using there may be some specific suggestions.
Another thing you can do is either right click on the task bar, or ctrl/alt/del together and select task manager, it will show running processes, and what is using most resources cpu / memory. It can be helpful is you see something at the top of the list, ideally cpu should not be 100%, that does indicate a problem.