I totally agree with you, free VPNs are usually ok for simple and rare surfing the net but on the other hand they have many cons too. First of all, it's slow speed which you mentioned above, sometimes it's really awful, especially in case of watching videos for example. They provide rather a few virtual locations too choose and it can be a real problem when you want to get access to certain resources. Also in case of free VPNs your constant payment will be always not your money but annoying and stupid advertisements. And keep in mind that anti-VPN software becomes better too especially in such countries like China, that's why free ones don't work there at all.
Comparing with them paid VPNs have such pros:
- better general security (but always check firstly the privacy policy of a certain provider, not all of them are really totally safe)
- faster speed
- better protection from VPN blockers (but this depends on a certain type of block, not all of them can overcome it, for example as the best ones in this case are considered ExpressVPN, KeepSolid VPN Unlimited https://www.vpnunlimitedapp.com/en/extras/personal-vpn-server, NordVPN and so on)
- support for all types of platforms
- unlimited bandwidth on all VPN servers