Internet speed in Portugal is on average 12.1 Mbps which is 15% slower than in United States (14.2 Mbps).
WiFi coverage in Portugal is high. Many hotels, hostels, cafes, restaurants and bars have it.
Avoid any public or free WiFi hotspots as someone could be sniffing all of your internet traffic. Instead, use only legitimate hotspots that are protected by a password. Make sure that you connect to the right one because criminals are usually naming their WiFi hotspots like "Hilton Hotel WiFi" etc. counting that tourists will not spot a difference.
Do not check your online banking and credit card statements while connected to a WiFi. Instead use a mobile internet, either directly via smartphone or on a laptop via created hotspot on your smartphone.
Use public computers (library, cybercafe) only to visit websites that don't require you to log in an account like checking public transportation schedules, maps, attractions etc. as the computer could be loaded with malicious software that logs everything you type (including your passwords).
When connected to a WiFi, some laptops have enabled file-sharing option which will share some of your files to other devices on the network. Make sure that setting is disabled before connecting.
Consider setting up two-step verification for your most important accounts (Google, Facebook). This requires you to enter not just a password but a second code whenever you log in using an unfamiliar computer. That way even if your password is stolen, criminal still wouldn't be able to log into your account.
Note that second code is usually received via SMS which is a problem while travelling because either you'll have another SIM card (and therefore number) or even if you have the old SIM - you still won't receive the code while you are abroad. The only way to solve it to switch from SMS code to one received within the app which is mobile-number independent (all major websites like Google, Facebook, Twitter etc. have that option).