OSX Yosemite bootable usb drive circle slash ("no" symbol)
The boot problem seemed to have been related to a disk error on the hard disk of the mac. I ran the disk utility and it detected a problem with the macbook's HDD (something with doubled file links), which it then repaired. The computer was still slow and unresponsive after the disk repair. After that the boot from the USB worked fine, and I was able to clean install OSX Yosemite.

I recovered the computer from my time machine backup, excluding applications, and now the computer is noticeably faster and more responsive. I don't know whether a specific application or accumulated OSX sludge was responsible for the slowdown, but I'm happy I went through the pain of the clean install.